A constructive response guaranteeing some conventionally agreed minimum strength. The term usually applies after partner has made a forcing opening bid (when the mere fact of keeping the bidding open would not promise strength).
For example, 2♣-Pass-2♠ would be a positive response. Requirements vary between partnerships. Some allow a positive response on a four-card suit; others require a five-card suit or even a five-card suit headed by two of the top three honours.