(1) A gap or space formed between the eyelids and eyeball.
(2) A fornix bound by the conjunctival membrane between the palpebral and bulbar conjuctiva of the eye.
The conjunctival sac enables the eyelids to move freely over the surface of the eyeball as the eyelids open and close, and into which the lacrimal fluid is secreted.
Word origin: conjunctival < conjunctiva: from Medieval Latin (membrāna) coniūnctīva, connective (membrane); sac: from L. saccus, "bag".
Synonym: saccus conjunctiva (Latin name).