The Collaborative International Dictionary
naked seed
Achene \A*chene"\, Achenium \A*che"ni*um\n. [Gr. 'a priv. + ? to gape.] (Bot.) A small, dry, indehiscent fruit, containing a single seed, as in the buttercup; -- called a naked seed by the earlier botanists. [Written also akene and ach[ae]nium.]
Wiktionary
naked seed
n. (context botany English) A gymnosperm seed which isn't enclosed in a plant ovary.