Definitions for "Tetrad"
The number four; a collection of four things; a quaternion.
A tetravalent or quadrivalent atom or radical; as, carbon is a tetrad.
Four grains united in a group.
A square 2 kilometres by 2 kilometres on the National Grid (4 square kilometres in area) often used as the unit for mapping in County Atlases. There are 25 Tetrads in a Hectad (10 kilometres by 10 kilometres) and they can be entered into the ' Grid Ref' field of the MapMate Site table using the so-called DINTY system where the Tetrads are given letters starting from the bottom left working up and across, so the most north-westerly Tetrad in the SW41 Hectad would be SW41E.
A tetrad is an area 2km x 2km square.
In Riemannian geometry, we can introduce a coordinate system over the Riemannian manifold (at least, over a chart), giving n coordinates
Keywords:  cardinal, sum, three, number, one
the cardinal number that is the sum of three and one