scalar

A single number, distinct from a vector. In `c · v`, `c` is the scalar and `v` is the vector; multiplying a vector by a scalar stretches (or shrinks, or flips) it without changing its direction. Loss values, learning rates, masses, and individual probabilities are scalars; positions, velocities, gradients, and feature embeddings are vectors. The word survives from a time when "scale" meant "stretch by a number" — exactly what scalar multiplication still does.