Final answer:
The hard outer covering of some dinoflagellates is called a theca, which is a complex structure made up of cellulose plates that provides protection and supports movement.
Step-by-step explanation:
The term for the hard outer covering of some dinoflagellates is theca. Dinoflagellates are known for their diversity in shape and size, and many have a complex cellulose armor known as a theca. This structure consists of several large cellulose plates enclosed in vesicles, just inside the outermost membrane, helping them move with a characteristic whirling motion due to their two flagella. The function of the theca is similar to that of a shell or exoskeleton in other organisms, providing not only protection but also aiding in the organism's mobility.