A fully enlightened Buddha. An Arhat has seen anatta, annica, dukkha, and dependent origination, both forward and backward.
This seeing is not ordinary knowledge but arises when purity has been cultivated and all four jhanas and the four immaterial attainments have been experienced, investigated, understood and remembered.
The status of Arhat is the highest ideal in Theravada Buddhism.
In Mahayana Buddhism, the Arhat is defined as one who enters Nirvana but is unable to teach others how to do so.
The Buddha recognized no such limitation on an Arhat so we can take the Mahayana position with a grain of salt.