Twenty-foot equivelant unit (TEU)
Used to measure a vessel's capacity by counting how many containers of 20' length can be loaded onboard. A vessel's intake capacity is composed of 'slots' meant to receive either 2x20'or 1x40' containers. Due to the curvature of some cargo holds some slots might be made exclusively for 20' while it is always possible to fit 2x20' in any 40' slot.
The 20' container is then the 'lowest common denominator'; so we will count the total amount of 20' that can be fitted in any slot to envision the total carrying capacity of vessels.