This number is a prime.
661 is the start of a record-breaking twin prime gap.
Lord Kelvin, one of Britain's great physicists, published 661 papers on a wide range of scientific subjects. [Rosen]
661 (the 11th star number) is the 121st prime (121 is the 5th star number). In the Hebrew Bible, Esther (אסתר), which means "star" (Strongs H635), has a gematria value of 661. [Slattery]