Accumulator
Beginner
The running total inside a math unit, where many multiplied pairs are added up.
Novice
The register that holds the running sum in a multiply-accumulate unit. It is usually much wider than the inputs (for example 32 bits for 8-bit inputs) so that adding thousands of products doesn’t overflow or lose small contributions.
Expert
Fixed-point (Kulisch-style, exact) for integer inputs, typically INT32 for INT8; floating point (FP32, sometimes FP16 or a reduced-width internal format) for float inputs. Its width and the length of the reduction set the accumulation error; long reductions are often split into chunks summed at higher precision.
Explained in Number formats (Architectures).
See also: MAC (multiply-accumulate), Swamping.