Negative free bid

Negative free bid is a bridge convention (or, more accurately, a treatment) whereby a free bid by responder over an opponent's overcall is not forcing, and just shows a long suit in a weak hand. This is in contrast with standard treatment, where a free bid can show unlimited values and is unconditionally forcing. The treatment is a relatively recent invention, and has become quite popular, especially in expert circles.
Negative free bids are supposed to solve relatively frequent situations where the responder holds a long suit with which he would like to compete for a partscore, but is deprived from bidding it by opponent's overcall. For example, if South holds:♠ 86 ♥ KJ10852 ♦ K6 ♣ 532, partner opens 1♦ and East overcalls 1♠, he couldn't bid 2♥ in standard methods, as it would show 10+ high-card points, and a negative double would be too off-shape. With NFB treatment in effect though, he can bid 2♥ which the partner may pass (unless he has extra values and support, or an excellent suit of its own without tolerance for hearts).
However, as a corollary, negative free bids affect the scope of negative double; if the hand is suitable for "standard" forcing free bid (10-11+ points), a negative double has to be made first and the suit bid only in the next round. Thus, the negative double can be made with the following types of hand:

• A weakish hand with unbid suits (unbid major)
• A stronger hand with unbid suits
• A strong (opening bid or more) one-suited hand.

This can sometimes allow the opponents to preempt effectively.

Responder's new-suit bid is NOT a NFB if the bid is made:

• At a level of 3H or higher. If partner opens 1S and your RHO overcalls 3D, no Negative Freebid is available; your new-suit bid of 3H or 4C is forcing.
• At the one-level. Responder's new-suit bid at the one-level carries the standard meaning. After 1D by partner, 1H by RHO, a freebid of 1S is unlimited, showing 6+ points and a 5+-card suit (since you would make a negative double if you held only 4 spades).