honorarium — A voluntary fee paid for professional services that are rendered nominally without charge

a distinguished guest lecturer shakes hands with a university department head while receiving an envelope. The professor smiles graciously as the envelope is placed in his other hand with a gesture of appreciation. The guest lecturer had just finished delivering a free educational talk to students. Now the university is offering a small token of gratitude for the valuable service provided without charge. Other faculty members stand nearby nodding approvingly at this gesture of respect. a voluntary fee being paid for professional services that were rendered nominally without charge