The term wet refers to the connectedness of the community cards. It's translatable to most all variants of poker depending on the game mechanics.
Specifically, there are three things to consider when judging the wetness of a board,
- Highness (Aces, Kings, Queens,...). The higher the cards, the wetter the board.
- Suitedness (Monotone or all one suit; Two-Tone; or 'Rainbow' noting all different suits).
- Connectedness (The proximity/coordination of the cards in rank to each other). A board of Ace, King, Queen, is very coordinated. A board of King, 7, 2, is wholly uncoordinated.
There are nuances regarding wetness. Wetness is relative to the opponents range. This is important when considering how to proceed against specific opponents, as the same board isn't wet for all types of opponents. For example, vs a tight opponent, a board consisting of high, suited, connected cards is wet, but not necessarily so for an opponent who is very loose. This is after consideration of the width of an opponents likely range.
A wet board is a board with many possible straights, flushes, straight and flush draws, and 2 pair hands that make sense.
For instance, a board of JcTd8d is very wet - there is a diamond flush draw, and hands like QJ, JT, T9, 98 are all very strong. Plenty of suited one and two-gappers also have a lot of equity.
In contrast, a flop of Ks9d6c is very dry - the only big draw is 78, and 2 pair is fairly unlikely (although many otherwise tight recreational players will play 69).
I know this has been answered, but I would like to offer a simple definition.
A wet board is one on which a single pair is not a significant favorite against a significant portion of a non-air range.
For example, on [Tc 9c 7d] continuing ranges that hit do not contain significant sub-ranges which are huge underdogs to any single-pair hand. This is not true for a board like [7h 2d 2c] on which hands like 66-33, comprising a fair amount of continuing hands, have poor equity versus a better pair.
I don't like defining wetness relative to a situation-specific range since it makes sense to speak about how your opponent perceives the board texture to have connected with your own range (the texture can thus be wet even though Villain's action-inferred range does not connect).