Pronunciation: (u-bun'duns),[key] —n. 1. an extremely plentiful or oversufficient quantity or supply: an abundance of grain. 2. overflowing fullness: abundance of the heart. 3. affluence; wealth: the enjoyment of abundance. 4. Physics,Chem.the number of atoms of one isotope of an element divided by the total number of atoms in a mixture of the isotopes.