a pig: the domestic swine, an animal counted unclean in the Law of Moses
The word names the ordinary pig, the same barnyard animal an English reader pictures. In the Gospels it turns up almost entirely in two scenes: the large herd that stampedes into the lake after the demons enter it, and the pigs the runaway son is reduced to feeding when his money runs out.
For a Jewish reader the animal carried a charge that is easy to miss today. The Law of Moses declared the pig unclean and forbade eating it (Leviticus 11:7; Deuteronomy 14:8), so it became the stock picture of what is impure. That background is what gives each scene its edge: a herd of two thousand pigs marks the setting as heavily non-Jewish country, and a son of Israel hired to feed pigs, and hungry enough to want their food, has sunk about as low as a person could. The Hebrew word behind those food laws is חֲזִיר.
The one proverbial use, in Matthew 7:6, leans on the same reputation: "do not give what is holy to dogs, and do not throw your pearls before swine." Here the pigs stand for people who cannot recognize what is sacred and will only trample it underfoot.