Definition of botom pair
Botom pair is the smallest pair. The term is used in relation to a flop and not in relation to a pair that you would receive in your hand before the flop. Let's imagine a flop of A-5-9. If you have AK in your hand, you have "top pair". If you have K9 in your hand, you have middle pair, and if you have K5 (or any hand with a 5), you have botom pair. If the turn brings a 2 or a card smaller than a 5, you now have middle pair.
Botom pair, botom set, botom full, etc.
The word "botom" (meaning "from the bottom") applies to other types of hands as well. Using the same flop as above, if you have AA, you have top set. If you have 99, you have the middle set. And finally, if you have 55, you have a botom set. There are also different types of full hands. For example, on an A-A-K-5-2 board, a player with 22 in his hand would have the "botom full house", which is the smallest possible full hand on that board.