Slideshows Images Quizzes
font size

Medical Definition of Sinus

Sinus: 1. An air-filled cavity in a dense portion of a skull bone. The sinuses decrease the weight of the skull. The sinuses are formed in four right-left pairs. The frontal sinuses are positioned behind the forehead, while the maxillary sinuses are behind the cheeks. The sphenoid and ethmoid sinuses are deeper in the skull behind the eyes and maxillary sinuses. The sinuses are lined by mucous-secreting cells. Air enters the sinuses through small opening in bone called ostia. If an ostium is blocked, air cannot pass into the sinus and likewise mucous cannot drain out. See also: Sinusitis.

2. A channel permitting the passage of blood or lymph fluid that is not a blood or lymphatic vessel, such as the sinuses of the placenta.

3. A tract or fistula leading to a cavity which may be filled with pus.

The word was borrowed from the Latin noun "sinus," which means "curve, fold, or hollow." The same root gave rise to "sinuous".

Source: MedTerms™ Medical Dictionary
Last Editorial Review: 5/13/2016

Drug Medical Dictionary of Terms by Letter

Top RxList Drug News

Use Pill Finder Find it Now See Interactions

Pill Identifier on RxList

  • quick, easy,
    pill identification

Find a Local Pharmacy

  • including 24 hour, pharmacies

Interaction Checker

  • Check potential drug interactions
Search the Medical Dictionary for Health Definitions & Medical Abbreviations

Health Solutions From Our Sponsors