A formal complaint that a solicitation or award decision broke a rule, filed with the contracting agency, the Government Accountability Office, or the Court of Federal Claims. Protests can pause performance while they are decided, which is real leverage, but filing one without a genuine legal basis burns goodwill with a buyer you likely want to sell to again. Most protests come right after a debriefing, once you can actually see the reasoning you are challenging.

In a sentence

"Protests not based on alleged improprieties in a solicitation must be filed no later than 10 days after the basis of protest is known or should have been known." That clock runs from when you spotted the problem, not from the award date, so a decision to protest has to happen fast, usually with a lawyer's help.