What is an mtDNA transition? What is the nomenclature for displaying them?

Transitions are types of DNA mutations. They are places in your DNA where nucleotides (Cytosine, Guanine, Adenine, and Thymine) have changed value.

Transitions are where a purine has mutated to the complimentary purine (A <-> G) or where a pyrimidine has mutated to the complimentary pyrimidine (C <-> T). Transitions are shown by giving the original value capitalized before the location and the mutated value capitalized after the location. Thus, a transition of a C nucleotide at locus 146 to a T is shown as C146T.