ADP (Average Draft Position): The average spot in the draft where a player is being drafted. An ADP of 12.0 means that player is being drafted on average 12th overall. It is important to make sure that you are looking for the ADP that applies to your league's scoring (e.g., PPR, non-PPR), because a player's ADP will fluctuate based on those rules.
Bench players: Players on your team who are not in your starting lineup for a given week. Their production will not count toward your total score.
Best-ball scoring: A format in which the manager doesn't need to set his/her lineup, rather it is determined by taking the highest-scoring players at each available lineup spot.
Boom-or-bust: A player who has a high ceiling, a low floor and is usually inconsistent on a week-to-week basis. This often refers to players who may not receive a heavy workload, but have a tendency to make big plays and score long touchdowns.
Breakout: A player who has made a significant leap in production compared to what he had done previously in his career.
Bust: A player who has not lived up to your expectations. A player can be a bust in a given week or for the season as a whole.
Bye week: Each NFL team takes one week off during the 18-week NFL season. This is called their "bye week." Some fantasy leagues also grant a bye week to their teams.
Ceiling: The maximum statistical upside a player has for a week, season or career. A high ceiling means the player has the potential to score a lot of fantasy points. A low ceiling means he sports little upside. Also see: floor.
Cheat sheet: Rankings used for fantasy drafts. May also include other helpful things like injury reports, depth charts, sleepers, busts and breakouts.
Commissioner (Commish, League Manager, LM): The Commish is responsible for creating the league, setting the scoring, determining the roster requirements and managing the league as the season moves along, including settling any disputes.
Cut, drop, release: Each can be used interchangeably to refer to a player who you no longer want on your team. You cut/drop/release the player for a free agent or via waivers.
Deep league: Typically, leagues with more than 12 teams.
Depth chart: The hierarchy within each NFL team at a given position -- e.g., starting quarterback, backup quarterback, third-string quarterback.
Draft: How each team in your league builds its respective roster of players. Also see: snake drafts and salary cap drafts.
Dynasty league: A long-term league where you carry over most of your roster from the end of one season to the beginning of the following season.
FAAB (Free Agent Acquisition Budget): A set amount of units each team has to spend on free agents during the season, typically done by blind bids, with the highest bid adding the player to its roster.
Flex: A utility roster spot. Typically, such a spot can be filled by a running back, wide receiver or tight end, though some leagues also include other or fewer positions in their flex spot(s). Also see: superflex.
Flier (flyer): A player who is drafted or added as a free agent because there is an outside chance that he may exceed expectations for that week or a season. You may also hear this referred to as a "lottery ticket" or "dart throw."
Floor: The lowest expected statistical production a player has for a week, season or career. A high floor means the player should provide consistent fantasy production. A low floor means he carries a notable risk of doing little or nothing statistically. Also see: ceiling.
Free agent: A player who is not currently rostered by a team in your league.
Insurance player: Rostering the primary backup to one of your better players. Managers most frequently seek "insurance" at the running back position. It's a protection policy that allows you to replace your star player with the player most likely to step into the vacated role should an injury/suspension occur.
IDP (Individual Defensive Player) leagues: These leagues use the stats produced by NFL players on the defensive side of the ball, usually in place of defense/special teams (D/ST) units.
Injured Reserve (IR): For fantasy leagues, this is a roster spot that you can use to stash an injured player. In many leagues, such a player also must be on his actual NFL team's injured reserve list or officially listed as "out" for that week's game.
Keeper league: A fantasy league in which you may carry a certain number of players from the previous season into the next season. Similar to dynasty leagues, but typically with fewer players kept.
Mock draft: A practice draft. Helpful for trying alternate strategies and preparing yourself for different circumstances that arise, including draft slots.
Manager: The person in control of all decisions (draft, weekly starters, trades, free agency, etc.) for a fantasy team.
Pickup: A player you add from the free-agent pool or via waivers.
PPR (point per reception): A league which rewards a player one fantasy point for each reception (catch). Many leagues use "1/2 PPR," which rewards 0.5 points for each reception (catch).
Projections: Educated guesses as to what stats a player should produce in a given week or season.
QB1, QB2: In a 10-team league, a QB1 is a quarterback who ranks as a top-10 option, while a QB2 is ranked from 11-20 at the position.
RB1, RB2: In a 10-team league, an RB1 is a running back who ranks as a top-10 option, while an RB2 is ranked from 11-20 at the position.
RBBC (running back by committee): A backfield where two or more running backs share the workload, rather than when there is a featured back, who garners the majority of the workload.
Roster: The players that make up a fantasy football team. These are the players you see on your "My Team" page.
Salary cap draft: A draft in which each manager uses as predetermined budget to build a team by bidding on players, with the highest bid securing each player during the draft.
Sleeper: A player who you think will exceed his draft spot.
Snake draft: A draft where the team who drafts first in Round 1 will draft last in Round 2. The team drafting last in Round 1, drafts first in Round 2.This snaking approach in a 10-team league would look like this: 1-10, 10-1, 1-10, etc.
Stacking: The practice of starting multiple offensive players from the same team. This is most commonly done by pairing a quarterback with his top wide receiver. It is a high-risk, high-reward strategy aimed to take advantage of favorable matchups and concentrated usage patterns.
Streaming: Using the waiver wire/free agency to fill a roster spot each week, rather than locking one player into that spot for the long term. This approach is most commonly used for the D/ST roster spot.
Superflex: A flex roster spot (OP in the ESPN Fantasy game) that allows managers to start a QB, RB, WR or TE. Because you can start two QBs in a superflex league, quarterbacks are generally drafted much higher than in a traditional one-quarterback format. Also see: flex.
TE1, TE2: In a 10-team league, a TE1 is a tight end who ranks as a top-10 option, while a TE2 is ranked from 11-20 at the position.
Trade: A swap of players and/or draft picks between two teams.
Transaction: Any change to your roster, such as trades, free-agent pickups, etc.
Undroppables: A list of elite players who cannot be cut in ESPN leagues (if your league decides to use this feature). The list is updated by ESPN Fantasy as the season progresses and is meant to preserve the integrity of the league.
Waivers: A player is on waivers after being dropped by another team. When on waivers, the waiver order of the interested teams determines which roster he joins. Should the player clear waivers (that is, no manager labels him as worth their spot in the waiver line), he is added to the free-agency pool and, thus, available for the first team that wants his services.
Waiver order: The order in which your league's free-agent waivers are processed.
WR1, WR2, WR3: In a 10-team league, a WR1 is a top-10 wide receiver, a WR2 is ranked from 11-20, and a WR3 is ranked 21-30.
Zero-RB strategy: A strategy wherein a manager opts against selecting any running backs during the first four or five rounds of a draft. It is based on the belief that running back is the most volatile and injury-prone position. Managers who use this strategy typically load up on high-volume wide receivers early in the draft, then take "fliers" on high-upside running backs in the later rounds or plan to address the position in-season via trade or the waiver wire.
For NFL-specific phrases, check out the ESPN NFL Statistics Glossary.