A co-dom is a dominant that shares the dominant role with another person. A co-dom is also sometimes referred to as a switch or a co-top.
More About Co-Dom
A co-dom, or a switch, participates in BDSM activities in both roles. Sometimes, they act as a submissive and others as a dominant. Depending on the relationship the switch is either a co-dom with their partner, or is submissive in one relationship and a dominant in another.