Noun, pl. animals
a. being eukaryotic (i.e. the cell contains a membrane-bound nucleus) and usually multicellular (unlike bacteria and most protists, an animal is composed of several cells performing specific functions) ;
b. being heterotrophic (unlike plants and algae that are autotrophic, an animal depends on another organism for sustenance) and generally digesting food in an internal chamber (such as a digestive tract);
c. lacking cell wall (unlike plants, algae and some fungi that possess cell walls);
d. being generally motile, that is being able to move voluntarily;
e. embryos passing through a blastula stage;
f. possessing specialized sensory organs for recognizing and responding to stimuli in the environment.
2. A living organism that feeds on organic matter, typically having specialized sense organs and nervous system and able to respond rapidly to stimuli. (dictionary.com)
1. Of or relating to animals (biology-online.com)
Word origin: Middle English, from Latin, from animāle, neuter of animālis, living, from anima, soul.