Steam is the whole market moving at once, and it is over fast.

Steam, in plain words

A steam move is a sudden line move that runs across many sportsbooks within seconds or minutes. It usually starts when respected money hits one or two books that move first. The rest follow, because no book wants to be the last one offering the old number.

Steam is information about price, not a tip about the result. The useful part is the speed: the first books move, then a gap opens while slower books still show the old line. That gap closes quickly. By the time a move is visible everywhere, the best of it is usually gone.

Chasing steam late means taking the new number at the books that already moved, which is often the worst price of the night. Read it the other way instead: if a line you already hold steams in your direction, you probably beat the close. That is the signal worth tracking, and it is what closing line value measures.