COTS = Commercial off the shelf (stuff you could pick up in a Radioshack before they became mini-Bestbuys, basically).

RCS = Radar Cross-section (a bit more field-specific, more or less how "big" the thing looks on radar, but it is a bit jargony -- it depends on things like the material and radar wavelength).

COTS -> Commercial off-the-shelf

RCS -> Radar cross section

COTS: Commercial off the shelf, stuff you can buy easily.

RCS: Radar Cross Section - how big of a reflection an object makes at radar wavelengths.

"Customer Off The Shelf" and "Radar Cross Section"

Commercial Off The Shelf

Radar Cross Section