MIDVOYAGE is a term used to describe the middle of a voyage or journey. ASEA is an acronym for 'At Sea, ' which is used to describe a vessel that is in the middle of a voyage, usually on the open ocean. ASEA is also used to describe a vessel that is not in port or harbor. MIDVOYAGE is often used to describe the position of a vessel when it is in the middle of a voyage and has not yet reached its destination.