That one cannot decline; unavoidable. / (grammar, of a word) Not grammatically declinable.

An allotrope of carbon, consisting of planes of carbon atoms arranged in hexagonal arrays with the planes stacked loosely, that is used as a dry lubricant, in lead pencils, and as a moderator in ...

plural of markerboard

A relatively low-status woman in the sociosexual hierarchy, (ranking below alpha, beta, and gamma females, but above omega females). / A female animal that ranks fourth in the social hierarchy.