Mafic is a geological term meaning rich in magnesium and iron. It is a portmanteau of magnesium and ferric. Mafic describes rock, magma and silicate minerals which are higher in the heavier elements.
Mafic minerals have a specific gravity greater than 3.
Through a cooling process called Bowen's reaction series, what is mafic becomes felsic—meaning that it ends up having a higher concentration of the lighter elements. At the middle of this process the rocks are known as intermediate.
Common mafic rocks include basalt and gabbro and common mafic rock-forming minerals include olivine, biotite mica, clinopyroxene and amphibole.
Soon to be Doctor of Geology Auduster says: "mafic" as a categorisation only really applies to igneous rocks, And that is generally shorthand for "lots of dark minerals".