Quick Definition
The narrow fleshy area on the ventral surface of the head between the gill openings; whether or not it is connected to the body is a diagnostic character.
📖 Full Definition
The isthmus is the narrow ventral portion of the head between the two opercular (gill) openings. Whether the branchiostegal membranes are joined to each other across the isthmus (and thus joined to the body wall) or are free (united only with each other) is an important character in distinguishing fish families. For example, in trout the membranes are free from the isthmus; in many perch-like fishes they are attached.