Official term for latitude longitude coordinate system?

A standard term is geodetic coordinate system. However, the market leader in GIS software tends to invent new terminology with each new release of its flagship system, and by virtue of gaps in fundamental training by most of its users (or just plain enthusiasm for new words), this terminology quickly gets adopted by the majority. For example, "Geographic coordinate system" was newly introduced with ArcGIS 8 about ten years ago but AFAIK it was not in common use before then.

I like to use Plate Carrée:

The equirectangular projection (also called the equidistant cylindrical projection, geographic projection, or la carte parallélogrammatique projection, and which includes the special case of the plate carrée projection or geographic projection) is a simple map projection attributed to Marinus of Tyre, who Ptolemy claims invented the projection about AD 100.

WGS84 is the coordinate system used when giving latitude and longitude, such as a coordinate from a handheld GPS device.