Bottom pair is specifically when a player has a hole card that matches the lowest card on the board, thus making bottom pair.
How Strong is Bottom Pair?
In Hold’em, bottom pair is a relatively weak made hand, albeit one which can occasionally be ahead of the opposition. However, even when this is the case, it’s difficult when possessing bottom pair to evaluate the situation with enough accuracy and confidence to commit. Of course, there can be times when bottom pair serves as a bluff-catcher, but in general it would be too risk put a significant amount of chips on the line. Furthermore, it is prudent to settle for seeing a showdown with bottom pair rather than turning such a vulnerable hand into a bluff. It should be said that bottom pair is very weak in Omaha.
Note that there is a distinction to be made between a bottom pair and an underpair. The former can be made only by pairing a hole card with the lowest card on the board, while the latter is a pocket pair that ranks lower than the board.