What is a "decade" as a unit of measure (ex. a decade of the EM spectrum)?

While many units are available for physical measurables, there are only a few that identify unitless variables, like ratios.

One is 'octave', meaning a factor of two (usually in frequency); another is 'decade', meaning a factor of ten. A third is bel, which grows a suffix from time to time, and indicates (almost always) a factor of ten in power. The tenth-of-a-bel, decibel, is denoted 'dB'.

Percent, parts-per-million, pH, are also unitless.

Any attempt to line-fit data on log/log or semilog plots will involve one or more axes being unitless, and give rise to phrases like 'dB per octave'. When your data is spread over a 100:1 range, it might look best on two-decade semilog paper.

From 10Hz to 100Hz is a decade (on a logarithmic axis this is $10^2$ to $10^3$).

A decade is a factor of $10$, so it's a way of assigning a unit to the common logarithm ($\mathrm{log}_{10}$). It's also frequently assigned the unit symbol $\mathrm{dex}$.