Mean Reciprocal Rank
A ranking metric that rewards an early first relevant result using reciprocal rank and averages the score across queries.
MRR is useful when a user mainly needs the first correct result. If the first relevant item appears at rank two the contribution is 1/2; at rank five it is 1/5. Additional relevant results do not receive separate credit.
That makes MRR a natural fit for first-answer retrieval tasks, but an incomplete metric when many relevant documents must be ranked well.