Instress is Gerard Manley Hopkins's term for the force or energy that makes a thing what it is and lets a reader perceive its unique inscape. It is the inner pressure, pattern, or vitality that gives a natural object, person, or moment its distinct identity.
Hopkins used the word to describe more than visual appearance: instress suggests the living energy behind form, the quality that a poem may try to capture through sound, rhythm, and exact observation.
Example
The apple glowed with its own quick law,
red as a held-in flame;
the poet felt its instress there,
its pulse, its hidden name.