a long narrow opening
"The cat squeezed through the small slit between the fence boards to escape."
a narrow fissure
"A thin slit appeared between the old wooden floorboards where someone had kicked them apart years ago."
A narrow cut or opening; a slot.
"She carefully sliced a thin slit in the packaging to slide her finger through and open it."
To cut a narrow opening.
"The mechanic used a razor to slit the old tire and let out all the air."
Having a cut narrow opening
"The surgeon carefully slit the incision to ensure it remained a precise, narrow opening for the procedure."
Slit comes from Old English slītan, meaning "to tear apart," which traces back to the Proto-Indo-European root *(s)leyd-. It is a doublet of the word slite and is also related to the French borrowing slice.