Poker theory is generally based on mathematical principals to define the theory.
The term generally came to popularity after 1987 when David Sklansky published "The Theory of Poker".
From Two Plus Two:
"Theory of Poker by David Sklansky discusses theories and concepts applicable to nearly every variation of the game, including five-card draw (high), seven-card stud, hold'em, lowball draw, and razz (seven-card lowball stud). This book introduces you to the Fundamental Theorem of Poker, its implications, and how the theorem should affect your play."
The term is widely used. The use of the tag should imply that the question is focused on a discussion of particular situations, in the context of game play.