Reduced-round attack
An attack on a cut-down version of a cipher, with fewer rounds than the real thing, measuring how much margin is left.
Modern block ciphers repeat a round function a fixed number of times, and part of the security margin is measured by how many of those rounds can be broken. A reduced-round attack targets a variant with fewer rounds than the original. The result shows how much margin remains; it does not show that the full cipher is broken.
AES is the standard example. The best known attack on full AES-128, the 2011 biclique attack, needs roughly 2^126 operations: a little under brute force and of no practical consequence. A break of a reduced version is a milestone for the field. Calling it a threat to AES is a different claim altogether.