Flush
A flush is five cards of the same suit, which beats a straight and loses to a full house.
Suits have no rank against each other, so two flushes are compared card by card from the top. That is why holding the ace of the suit matters so much: it is the one card that cannot be beaten by another flush.
It is rarer than it feels. Of all 2,598,960 five card hands, 5,108 are a flush, which is 0.197%, and it is common precisely in the situations where people go looking for it because they started with two cards of a suit.
an example
your cards
board
Three hearts on the board and two in your hand give you the ace high flush. Any other player with two hearts also has a flush, and none of them can beat yours, because the ace is in your hand.
This example is made up. The real one is in your own hand history: find flush in your own hands and see how often it came up and what it cost you.
related terms
Flush draw
A flush draw is four cards of one suit with more cards still to come, so one more card of that suit completes a flush.
Straight flush
A straight flush is five consecutive cards all of the same suit, the best hand class there is.
Nuts
The nuts is the best hand possible on a given board, the one no other two cards can beat.
next
Knowing the word is not the same as seeing it.
All ten hands in order, with how often each one turns up.