The word 'zooplankton' is a modern scientific compound formed from two Greek roots. 'Zoo-' derives from the Greek 'zōion', meaning 'living being' or 'animal', which itself traces back to the Proto-Indo-European root gʷeyh₃- meaning 'to live'. 'Plankton' comes from the Greek 'planktos', the neuter passive participle of 'plazein' meaning 'to make wander or drift', related to the PIE root plak- ('to strike, to hit'). The term was coined in the late 19th century to logically categorize the drifting component of aquatic life that consists of animals, as distinct from 'phytoplankton' (plant-like drifters). Thus, the word's literal meaning, 'drifting animal', perfectly captures its modern biological definition.

Structure: zoo (animal) + plankton (that which drifts) - zoo: From Greek 'zōion', meaning 'animal'. In this compound, it functions as a combining form meaning 'pertaining to animals'. - plankton: From Greek 'planktos', meaning 'drifting' or 'wandering'. It functions as the core noun, describing the drifting organisms.