Ordinary mirrors are found in bathrooms, dormotories and on corridors in Hogwarts.
There is a mirror in corner of the dormitory, as Harry rushed to it to look at himself with his body invisible in the invisibility cloak (PS12). Ron used it to look at himself in his dress robes before the Yule ball (GF23).
Mirrors are also found in bathrooms, such as Moaning Myrtle's bathroom (CS9), where an old, cracked, spotted and large one hangs. This one is mentioned most often in canon. Another old one is found in the boys bathroom (HBP21).
There also used to be a secret passage out of Hogwarts behind a mirror on the fourth flour (PA10). However, the passage was blocked in Harry's second year, according to Fred.
Middle English: from Old French mirour, based on Latin mirare 'look at'. Early senses also included 'a crystal used in magic' and 'a person deserving imitation'. (Oxford Dictionary)