A signal can be characterized by how much squared magnitude it accumulates over time.

For a continuous-time signal $x(t)$, its energy is

$$E_x=\int_{-\infty}^{\infty}|x(t)|^2,dt.$$

Its long-term average power is

$$P_x=\lim_{T\to\infty}\frac{1}{2T}\int_{-T}^{T}|x(t)|^2,dt,$$

when the limit exists.

For a discrete-time signal $x[n]$, the corresponding definitions use sums:

$$E_x=\sum_{n=-\infty}^{\infty}|x[n]|^2,$$

$$P_x=\lim_{N\to\infty}\frac{1}{2N+1}\sum_{n=-N}^{N}|x[n]|^2.$$

A finite-duration pulse can have finite energy and zero long-term average power. A nonzero periodic signal instead has infinite total energy but finite average power.

These are mathematical signal measures. When $x$ represents a voltage or current, physical power also depends on the impedance used to interpret that signal.