A ruminant mammal with antlers and hooves of the family Cervidae, or one of several similar animals from related families of the order Artiodactyla.
"The spotted deer grazed quietly in the forest clearing."
In plain English: A deer is any of several hoofed mammals known for their graceful movement and often having antlers on males.
"A deer was grazing peacefully in the meadow when we drove by."
Usage: Use deer as an uncountable noun when referring to the animal in general, but add a number before it if you are counting individual animals (e.g., "three deer"). Do not use the word with articles like a or the unless specifying which particular group of deer is being discussed.
The word deer comes from Old English dēor, which originally meant "animal." It ultimately traces back to the Proto-Indo-European root for "breath" or "living thing."