Zoning codes sort uses into those permitted outright, those not allowed, and those in between. A conditional use sits in between: the code allows it, but only if the project meets conditions written for that use, such as a minimum lot size, a buffer from homes, limited hours or a cap on size.
The term means different things in different places. In some codes a conditional use is decided administratively: if the written conditions are met, staff approve it without a hearing. In others, a conditional use permit goes to the planning commission, the board of zoning appeals or the governing body, which can add conditions of its own. In those places it works much like a special exception.
The difference matters for timing and risk. An administrative conditional use is close to by right once the conditions are met; one decided at a hearing carries notice periods, a meeting calendar and the chance of denial.
The code's own definitions and its use table say which kind applies. In Charleston County, South Carolina, for example, the use table marks some uses as conditional, approved when the code's listed conditions are met, and marks others as special exceptions that go to a board hearing.