What does "transition" mean?

Additional senses

1. 2.(Rhet.) A passing from one subject to another. [He] with transition sweet, new speech resumes. Milton.
2. 3.(Biol.) Change from one form to another. Note: This word is sometimes pronounced tran*sish"un; but according to Walker, Smart, and most other authorities, the customary and preferable pronunciation is tran*sizh"un, although this latter mode violates analogy. Other authorities say tran*zish"un. Transition rocks (Geol.), a term formerly applied to the lowest uncrystalline stratified rocks (graywacke) supposed to contain no fossils, and so called because thought to have been formed when the earth was passing from an uninhabitable to a habitable state.