The state of being neither fully flaccid nor fully erect in the male penis. Most often used in the sense of moving towards an erection rather than loosing one.
Semi-erect penis - not yet suitable for entry into the female vagina or male anus, but suitable (and some would say preferably) erect for the purpose of fellatio.
Not to be confused with limp cock or impotence - a prehardon is thestage before the hardon or erection occurs.
I had a prehardon all night knowing that he was going to take me to his place and fuck my arse all night.
She kept flashing her tits at me and I either had a hardon or prehardon all night.
See also hardon.