a type of organism that has one or more cells each with a separate nucleus (= central part) containing chromosomes, which includes all animals and plants

"Animal" is a name reserved for multicellular eukaryotes—organisms that are bigger than a single cell and whose cells have a nucleus. “动物”是多细胞真核生物的专用名词，即比单细胞更大，并且细胞内有细胞核的生物。

Though giardia is a single-celled organism, it belongs to the eukaryotes. 虽然梨形鞭毛虫是一种单细胞生物，但它属于真核生物。