Final answer:
A transition mutation is a type of point mutation in biology where a purine is replaced by another purine or a pyrimidine is replaced by another pyrimidine.
Step-by-step explanation:
A transition mutation is a type of point mutation that occurs when a purine (adenine or guanine) is replaced by another purine, or when a pyrimidine (cytosine or thymine) is replaced by another pyrimidine. It is a type of nucleotide substitution, where one base is replaced by another. For example, if an adenine is replaced by a guanine, or if a cytosine is replaced by a thymine, it would be considered a transition mutation.