TEU definition from this page

TEU (Twenty Foot Equivalent Unit) is the unit of the capacity of a container ship, a container terminal and the statistics of the container transit in a port.
The two most common international standardised containers are those of twenty and forty foot. Depending on whether the ship loads twenty or forty-foot containers or a combination of the two, the number of containers held onboard the ship will differ. So as to express the capacity of a container ship in a uniform manner, the number of containers that the ship can load is converted into a number of containers of the smallest size, i.e. those that are twenty foot in length.
A forty-foot container is regarded as two twenty-foot containers or 2 TEU. How many TEU a container is, is calculated by dividing the container's actual length by twenty. In terms of costs, reference is often made to costs per TEU, which means that these costs will double for 40Ft units.