Definitions of Bad Beat

[HoldemPokerPlyr]

A hand being beat by another hand that had a very low percentage of becoming a winning hand.
www.gambleversity.com/poker/poker-glossary-term.php

When a hand is beaten by a lucky draw.
pokerworks.com/poker-dictionary/B

When one of the premier hands gets beat (ie a four of a kind gets beat by a straight flush). Sometimes players also use the term “Bad beat” when they get beat by a lucky draw.
www.buypokerchips.com/world-poker-tour-2.htm

When a player has the best hand but is beaten by an unlucky draw. It's worth remembering that poker has an element of luck, and no matter how good a player you are, any pre-flop hand can be beaten by any other hand after the flop.
www.love-texas-holdem.com/poker-glossary.html

-HoldemPokerPlyr