What is 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 achænium.] called?

achenium — 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 achænium.]. (Webster's 1913.)