Definitions for "Lamarckism"
The theory that structural variations, characteristic of species and genera, are produced in animals and plants by the direct influence of physical environments, and esp., in the case of animals, by effort, or by use or disuse of certain organs. It is a discredited theory, not believed by modern biologists.
An evolutionary theory (proved false by Darwin) stating that species change over time by the use and disuse of structures and the inheritance of acquired traits.
A theory of evolution whereby change occurs based on the use or disuse of a particular feature.    This is mostly just historical although more recently variants of this are making a comeback.