That's a term which has come into fashion lately, and it deserves to. Counterfactuals are a specific variety of BS, as our commenter notes.
The term can be used as a noun or as an adjective. The key to it is its conditionality (If...,then...might have...); the past subjunctive, combined with its lack of factual content. For example, "You bozo - you left a burner on. You could have burned down the house." Well maybe - but it did not happen. Thus no fact.
(An "indicative conditional," by way of contrast, is a past conditional which is founded on a real, factual consequence which occurred. For example, "If you bozos hadn't left the gate open, the dog would not have run into the street." Indicative conditionals are also debatable, due to their speculative nature, ie, cum hoc ergo propter hoc. For example, in my case, almost every time I water the garden, it rains afterewards.)
Counterfactuals are often used (abused) to make emotional arguments. "If the stimulus had been 3 trillion dollars, our unemployment rate would be 4%."
Free Dictionary offers this:
(Philosophy / Logic) expressing what has not happened but could, would, or might under differing conditions
(Philosophy / Logic) a conditional statement in which the first clause is a past tense subjunctive statement expressing something contrary to fact, as in if she had hurried she would have caught the bus.