Transitional: Change from one form, state or place to another.

Fossil: That which once was organic matter, now stone after undergoing petrifaction.

A transitional fossil is an intermediate fossil connecting different species in a taxonomic chain that alludes to the progression of macroevolution.

A fossil which came from a relative different enough from the fossil's own offspring to be categorized as different in species, hence supporting evolution because of the fossil's similarities between both its' lineage and progeny, connecting the generations by a display of evolutionary progression.