Thymine: A nucleobase that pairs with adenine in DNA but gets replaced by uracil when transcribing into RNA.
Pyrimidine: The category of nitrogenous bases that includes cytosine along with thymine (in DNA) or uracil (in RNA).
Complementary Base Pairing Rule: The rule stating that certain nitrogenous bases are always paired together - such as cytosines always pairing up with guanines.