endolymph

This term is a specialized anatomical descriptor used almost exclusively in medical, biological, and audiological contexts. It refers specifically to the potassium-rich fluid of the inner ear, distinguishing it from perilymph, which is the sodium-rich fluid surrounding the membranous labyrinth.

As a biological substance, it is treated as a mass noun. It describes a continuous medium rather than individual units, meaning it does not have a plural form in standard scientific usage.