Down Time
In Live Action Roleplaying it is quite common for player's to want their characters to perform actions outside of the main gaming sessions. For instance they might want to make a magical weapon, instead of acting this out they will inform a referee (GM) that this is what their character is doing. In these circumstances the character is said to be making the weapon in Down Time.
Down Time, as a term, is reserved for activities performed by characters outside of game sessions. It is used most commonly in Live games, but it can also appear in table top games. If game sessions tend to involve "adventures" occurring in a short space of game time (time as counted within the game), while a longer space of game time passes between sessions then characters may well perform down time activities. This is particularly true if the game involves a large number of players who will not want to spend a large part of the session working out what each character has been doing while alone, but want to get on with interacting with each other.
In this sort of situation a referee may well set time aside before or after the session to find out from each player individually what their character has been doing in Down Time.
Both these situations apply to Live Action games where there tend to be a lot more players than in table top games and real and game time tends to pass at the same rate while they are unusual in table top games. Hence the prevalence of the term in LRPs and its rarity in table top.