A gravitational wave's strain (GW strain, h) is a measure of its magnitude, specifically, the ratio (thus a dimensionless number) by which lengths are stretched or compressed. The term strain might be used to refer to the observed strain or to the strain along the direction of the wave's displacement, and may be the "maximum" strain of the wave (its amplitude) or the instantaneous displacement that is oscillating.
These relations are often cited:
h ≈ ΔL/L h ≈ ΔT/T
These are sometimes cited with "=" or "∼" instead of "≈", the latter of which is sufficiently loose to be strictly true whether referring to the observed strain or the wave's actual amplitude. The relation based on T represents a shortcut when relative distances are measured by time intervals, such as when using pulsar timing (pulsar timing arrays).
A variant, the characteristic strain (hc), which is sometimes what is intended by strain, is often used for general discussions: it is a kind of averaging (i.e., like RMS, in that it doesn't merely average to "zero" over each wave period), which also has a component varying by frequency. It is often used when explaining relative detector ability and source strength over the gravitational wave spectrum.