Conductivity–temperature–depth instrument

A conductivity–temperature–depth instrument, known as a CTD instrument (or sonde, probe, or device), or simply CTD, is an oceanographic sonde, or probe, that provides measurements of electrical conductivity, temperature, and pressure. CTD stands for conductivity, temperature, and depth, pressure being closely related to depth and electrical conductivity to salinity.

Source: Wikipedia — Conductivity–temperature–depth instrument (CC BY-SA 4.0)