The Collaborative International Dictionary of English v.0.48:

Guna \Gu"na\ (g[=oo]"n[.a]), n. [Skr. guna quality.]
   In Sanskrit grammar, a lengthening of the simple vowels a, i,
   e, by prefixing an a element. The term is sometimes used to
   denote the same vowel change in other languages.
   [1913 Webster]