Frames
Minsky proposed representing knowledge as stereotyped situations: a frame has slots, some filled with defaults that hold until something contradicts them.
Why it matters
Defaulting became a norm of knowledge representation rather than an error, the first step toward non-monotonic inference.
First-order logic gave no way to say that a room usually has a ceiling and then retract it. A frame does: the value stands until it is contradicted. Schank's scripts, object-based representation and description logics all descend from it. Minsky gave no formal semantics, which became the subject of a long argument with the logicists.