Sunk cost is that cost which is incurred in past and it is unavoidable because any past decision or action cannot be reversed.

sunk cost

Sunk cost is that cost which is incurred in past so in decision making process any cost which is incurred in past has no effect on future events and due to which all fixed cost which are also incurred in past has no effect in future that's why it is not relevent for decision making process, so for decsion making point of view yes fixed cost is sunk cost.

A cost that has already been incurred and thus cannot be recovered. because it has already happened. Sunk costs are independent of any event that may occur in the future.

A cost that will not be affected by later decisions is termed a sunk cost.