n (chemistry) Any of a class of generally water-soluble compounds, having bitter taste, that turn red litmus blue, and react with acids to form salts.
n (geometry) The lowest side of a in a triangle or other polygon, or the lowest face of a cone, pyramid or other polyhedron laid flat.
n (topology) A topological space, looked at in relation to one of its covering spaces, fibrations, or bundles.
n (linguistics) A morpheme (or morphemes) that serves as a basic foundation on which affixes can be attached.