Celeriac is a large gnarled looking root also referred to as celery root or celery knob. The root is related to celery and has the flavor of celery and parsley. The peeled root can be sliced thin, julienned and used fresh in a salad, or the root can be boiled or steamed. Additionally, the root can be boiled and pureed with potatoes or cut into pieces and added to soups and stews.
You can purchase celeriac at your local grocery store typically year round although the peak season is in the winter. You can also check for it at your local farmer's market.