transition

tran·si·tion

noun
\tran(t)s-ˈish-ən, tranz-, chiefly British tran(t)s-ˈizh-\

Definition of TRANSITION

1

: passage from one state or stage to another; especially : an abrupt change in energy state or level (as of an atomic nucleus or a molecule) usually accompanied by loss or gain of a single quantum of energy

2

: a genetic mutation in RNA or DNA that results from the substitution of one purine base for the other or of one pyrimidine base for the other