inertian

Inertian is a rarely used English term that may function as an adjective or noun, but in modern science the standard terms are inertia and inertial. Consequently, inertian appears mainly in historical, philosophical, or stylistic contexts and is not considered a common technical term.

When it is used, inertian is typically understood as meaning “pertaining to inertia” or describing the inertial quality of a substance or system. Because the term is not standard, its precise sense varies by text, and in technical writing it is usually replaced by “inertia” or “inertial” to avoid ambiguity.

In physics, inertia is the resistance of any physical object to a change in its motion or rest; it is quantified by mass. An inertial frame of reference is one in which Newton’s first law holds. The adjective inertial is used for things related to inertia or to inertial frames, whereas inertian remains an uncommon, largely historical or stylistic variant.

Etymology: inertian is formed from inertia with the English suffix -an, and is regarded as rare or archaic in contemporary usage.

See also Inertia; Inertial frame of reference.