An empirical counterpart of a concept is called:

The empirical, measurable counterpart of an abstract concept is called a variable.

A variable takes different values and lets an idea be observed and measured in reality.

A construct is itself an abstract concept built for study, not its measurable counterpart.

A definition only states the meaning of a term and does not measure it.

A hypothesis is a testable statement about the relationship between variables, not a concept's counterpart.

Turning a concept into a measurable variable is the process known as operationalisation.