Dump Stat
A Dump Stat is roleplaying gamer slang for a character statistic with a particularly useless effect compared to the other characteristics. It is the opposite of a God Stat. Dump stats may be specific to a campaign (for example, an action-focused campaign where most interaction is of the fist-to-face variety will render a character's charming social skills mostly irrelevant), a GM's house rules, or can be present in the system RAW. Dump stats are often tied to a character's primary class or focus; combat-oriented characters tend to get little benefit from a high Charisma attribute, for example, whereas spellcasting classes who are usually expected to stay out of frontline combat likewise benefit little from having high strength.
When a dump stat is obvious as a result of class selection or rules, if possible, many players will use it to ensure they have high scores in their important abilities (either by placing their lowest score in that stat or by neglecting to improve it in points-buy systems). Some consider this to be powergaming, but the practice is very widespread. No matter how you feel about the practice, it's hard to argue that players should be forced to spend finite resources on something that provides them radically lower benefits than anything else you can spend on. Some systems have made level up models which mitigate this a little, making additional points of high attributes more expensive, while low ones are very cheap, at least allowing players to spend left over experience, rather than the full rate for them.
The traditional example is the Charisma stat from Dungeons & Dragons 3rd Edition. All the other stats contribute some sort of direct benefit beyond the skills and abilities directly associated with them: Strength decides melee ability and carrying capacity, Dexterity adds to Armor Class, Reflex saving throws, and ranged ability (along with some forms of melee combat), Constitution grants extra hitpoints and Fortitude saves, Intelligence gives out extra skill points, and Wisdom increases Will saves. Thus, unless the player wants to focus on having social skills or casts spells with it, Charisma is always the stat that incurs the least penalties for dumping.