pul·mo·branchia \|pəl(ˌ)mō+\noun (pluralpulmobranchiae) Etymology: New Latin, from pulmo- + -branchia 1.: a gill or similar organ so modified as to breathe air 2.: a book lung (as of a spider) • pul·mo·branchial\| ̷ ̷ ̷ ̷+\adjective • pul·mo·branchiate\“+\adjective or noun