Pump
A pump is a card played to temporarily "pump up" (increase) a character's strength. Generally, pumps are played during the Skirmish Phase, after assignments have been made. Most pumps are events like Flurry of Blows (2C5) , especially when they use a +2/conditional +4 formula.
Some characters, like Gimli, Son of Gloin (1R13) can be said to "pump" themselves, and some other card types, like Saruman's Ambition (1C133) , can be discarded to pump characters. Since most pumps are Skirmish: special abilities, they end at the end of that particular skirmish unless the card specifies otherwise (such as Savagery to Match Their Numbers (1R139) ).
Cards that grant a continuous bonus, like Hobbit Sword (1C299) , are generally not considered pumps. Cards like Enraged (6R26) that grant a temporary strength bonus that starts before the Assignment Phase could theoretically be called pumps, but they're so rare that they're not often referred to at all.