FESS means -

Functional endoscopic sinus surgery (FESS) FESS is a minimally invasive technique in which sinus air cells and sinus ostia are opened under direct vision. It is called functional because it aims to return the working of the sinus to normal. FESS is based on the hypothesis that the osteomeatal complex (maxillary sinus ostium, anterior & middle ethmoid Ostia, frontal recess, infundibulum and middle meatal complex) is the key area in the pathogenesis of chronic sinus disease. Minor pathological changes in the nasal mucosa in the vicinity of the osteomeatal complex may interfere with mucociliary clearance or with the ventilation of maxillary, ethmoid and frontal sinuses. The underlying principle of FESS is that the sinus mucosa will return to normal if adequate drainage is established. No attempt is made to remove the sinus mucosa; rather, it is allowed to return to normal and to resume its normal function. FESS has greatly reduced the indications for conventional operations like those of Caldwell-Luc, Frontal-sinus operations, and an external ethmoidectomy.