Force buy — a situation where a team spends almost all or all of its accumulated money on weapons and utility, even though there isn't enough money for a full setup (for example, rifles for everyone plus armor and grenades). This is a compromise between a full eco round, where almost nothing is bought in order to save up for the future, and a standard full buy.
The decision to force is usually made by the IGL (in-game leader, the player responsible for the team's tactical decisions) or the captain, based on the match score, the opponent's economy, and the importance of the round. A force buy is a risk: if the round is lost, the team's economy takes an even bigger hit, and the next round will most likely also have to be played on eco or a semi-force. But if the round is won, the team gets money from kills and the opponents' weapons, which sharply improves its economic situation.
A typical example is a team that has lost several rounds in a row, while the opponent already has a full buy and a lead in the score. Instead of the obvious eco, the team can take the risk and go out with cheap submachine guns or high-power pistols, adding a couple of grenades like a flashbang or a molotov, to get at least some chance of taking the round and disrupting the opponent's economy.
Force buy is often confused with a semi-force — a softer variant, where not everything is bought, and part of the team takes weapons while the rest carry over pistols. The term comes from the general in-game economy of round-based shooters, where money management is just as important as shooting, and it has become established in Russian-language esports slang almost without translation, simply with a Russian ending — "forsit'" (to force), "zaforsili" (forced it), "poshli na fors" (went for a force).
Breaking down force buys is a common topic in post-match analysis: commentators and demo reviewers look at how justified the decision to buy weapons was from the standpoint of the round's economy and score, and how it affected the team's subsequent play.