Spectrum (or spectral) efficiency measures how much useful data can be transmitted per unit of radio bandwidth, typically expressed in bits per second per hertz (bit/s/Hz).

In other words, it answers: “How many bits of information can I push through each hertz of spectrum every second?” For mobile networks like 5G, higher spectrum efficiency means you can serve more data (and often more users) using the same chunk of frequency, instead of needing more spectrum.