Final answer:
Protozoa, or protists, are single-celled organisms with a nucleus that play important roles in food webs as a source of nutrients.
Step-by-step explanation:
Protozoa, which are single-celled organisms with a nucleus, are known as protists in biology. The term protozoa comes from the Greek words 'protos' meaning first and 'zoa' meaning animal. Protozoa are eukaryotic organisms and play important roles in food webs, providing nutrients for other organisms.