From Glossary of Meteorology
(Abbreviated CTD.) An electronic instrument designed to measure very accurate, nearly continuous profiles of conductivity, temperature, and pressure (depth) in the water column.
It is usually lowered on the end of a conducting cable through which the real-time measurements are transmitted to a recorder on the ship. Today there are also self-contained instruments (i.e., with an internal memory). Salinity is computed subsequently from the conductivity and temperature measurements. Accuracies are typically ±0.005 psu, 0.005 K, and 0.15% full scale for conductivity, temperature, and depth, respectively.