From Coastal Wiki
Definition of Zooplankton:
Small floating or weakly swimming animals that drift with water currents and which, with phytoplankton, make up the planktonic food supply upon which almost all oceanic organisms ultimately depend.
This is the common definition for Zooplankton, other definitions can be discussed in the article
Zooplankton are heterotrophic (non-photosynthetic) planktonic organisms, including protists, small animals, and larvae, which exist within the water column. See Marine microorganisms.
Different types of zooplankton: see Marine Plankton.