Noah Webster's New International Dictionary of the English Language

1. (adv.) In a singular manner; in a manner, or to a degree, not common to others; extraordinarily; as, to be singularly exact in one's statements; singularly considerate of others.

2. (adv.) Strangely; oddly; as, to behave singularly.

3. (adv.) So as to express one, or the singular number.