An arihant is one who has destroyed the four obscuring karmas and attained omniscience while still embodied, the first of the five beings saluted in the fundamental Jain prayer.
Omniscience in the body
In Jain teaching the soul innate qualities, above all infinite knowledge, are obscured by karma, and the arihant is one who has destroyed the karmas that obscure knowledge, perception, energy and the like, attaining omniscience, kevala jnana, while still living in a body, so the arihant is a living liberated being who knows all things directly. The Tirthankaras, the teachers who establish the Jain path in each age, are arihants who also teach, but not every arihant is a Tirthankara.
First of the five
The arihant is the first of the five supreme beings saluted in the Navkar Mantra, the central Jain prayer, before the fully liberated siddhas, the ascetic leaders, teachers and monks, and the salutation of the arihant first reflects the debt owed to the omniscient one who, still embodied, makes the teaching available. The standing is not conferred but attained through the immense spiritual labour of destroying the obscuring karmas, the culmination of the ascetic path. At the end of the embodied life the arihant sheds the remaining karmas and becomes a siddha, a liberated soul freed from the body altogether, so the arihant is the omniscient being in its final embodied phase, the living summit of Jain attainment.