The stage of development of an insect between any two moults (or molts - why I prefer the British spelling, I don't know).
Many insects have multiple instars.
The pictures below are from this site, and are of two instars of the brown marmorated stinkbug (a common pest around here).
The word is also used in a recent episode of Nature, titled "Sex, Lies, and Butterflies" - well worth watching.
Etymology: Latin instar, image, form, likeness.