Tumescence:
During sexual excitement, the sex organs become engorged with blood, i.e. they become tumescent (from lat. tumescere: to swell). In females, the first and most obvious sign of sexual excitement and tumescence is the lubrication of the vagina. In response to effective stimulation, the vaginal walls begin to secrete a clear fluid which soon provides a moist coating for the entire vagina in preparation for coitus. Without such lubrication, the insertion of a penis into a vagina could be painful for both partners. (The corresponding sign of tumescence in males is the erection of the penis. In short, as the penis becomes ready to enter the vagina, the vagina becomes ready to receive it).