Complementary DNA, also known as cDNA, is a single-stranded DNA molecule synthesized from messenger RNA (mRNA). Synonyms for cDNA include copy DNA, reverse transcription DNA, and complementary RNA. Copy DNA refers to the process of copying the mRNA sequence into a DNA molecule, while reverse transcription DNA refers to the reverse transcription process of synthesizing cDNA from RNA. Complementary RNA, on the other hand, refers to the RNA molecule synthesized from cDNA. Other synonyms for cDNA include reciprocal DNA, anti-sense DNA, and hybrid DNA. Regardless of the synonym used, cDNA is an essential tool in molecular biology for studying gene expression and protein function.