Range
Not one specific holding but every hand the opponent could have arrived here with. His cards are rarely visible, so the arithmetic runs against the set, not against a guess.
A range is the list of every hand the opponent could hold having done everything he did. Before the flop the field opens about 373 of the 1,326 possible combinations from the button. Each further action crosses part of the list out: against a half-pot bet he would fold some hands and raise others, and only the ones he calls with stay in the set.
Playing against one hand you have assigned him is the most common mistake at the table. The opponent does not show his cards, so there is no hand, there is a set, and the question is always the same: what share of the set do I beat and what am I paying for it. The answer is computed by enumeration: every combination in the set against my hand on this board.
An example from session 8
K♠J♠ on J♣ 8♠ 8♥ against a button open. On the flop my hand beats 319 of 373 combinations, 85.5% of his range. The 9♥ turn brings in straight draws and the share drops to 79%. The 7♠ river completes a straight for any ten, 61 combinations, and the share falls to 66%. The same hand, three streets, three different numbers, and none of them could be reached by thinking about one hand of his.
The same in business
A counterparty does not show his cards either, and he too has a range: every reason he could have had for doing what he did. A client who went quiet has not «gone to a competitor»; he is in a set of a dozen states, each with its own share. Shrink the field of uncertainty first, then apply precise tools: the reverse order gives a precise answer to an invented question.
The whole hand is in session 8: “A move out of turn says something about you that you did not mean to say”.
The other terms are in the glossary.