For a digital link carrying information at bit rate $R_b$ through an occupied bandwidth $B$, the spectral efficiency is

$$\eta=\frac{R_b}{B}$$

in bits per second per hertz.

A larger value means more information is carried through each unit of bandwidth.

Spectral efficiency is not determined by bit rate alone. Two systems can transmit the same number of bits per second while occupying very different bandwidths, and two systems using the same bandwidth can carry different rates.

Increasing the number of bits represented by each physical symbol can raise bit rate without raising symbol rate, but it also makes the possible received signal states harder to distinguish at fixed transmitted power.

Spectral efficiency is therefore one part of a larger trade-off among bandwidth, required signal quality, redundancy and implementation complexity.