Entry 2 senses · 2 variants Webster, 1913 Decillionth · De·cil·lionth · IPA a. n. 01 a. Pertaining to a decillion, or to the quotient of unity divided by a decillion. 1. Pertaining to a decillion, or to the quotient of unity divided by a decillion. 02 n. The quotient of unity divided by a decillion. 1. The quotient of unity divided by a decillion.