The Conjunctival sac of the eye is the space bound by the conjunctival membrane that's between the palpebral and bulbar conjunctiva, into which the lacrimal fluid is secreted.
It's a closed space when the eye is closed and when the eye is open, the sac is open anteriorly through the palpebral fissure which is between your eyelids.
Here's an image showing the Conjunctival Sac of the eye where you put eye drops etc.
