Cache line (block)
Beginner
The chunk a cache moves in one go, often 64 bytes. Asking for one number brings its neighbors along too.
Novice
The fixed-size unit a cache stores and moves, usually 64 bytes. Each line has a tag saying which memory address it came from. Fetching a whole line at once is what makes spatial locality pay.
Expert
The unit of allocation, transfer and coherence. The address splits into tag, index and offset; offset bits = log2(line size). Larger lines cut compulsory misses and tag overhead but waste bandwidth on unused bytes and raise false sharing.
Explained in Caches and coherence (Architectures).
See also: Cache, Locality (temporal and spatial), False sharing.