Pad near the center of a baseball field that batters must step on after before stepping on 3rd Base. In a perverted person's book, its defined as touching other peoples inappropriate places.

Just remember the 4 F's kids:
1st Base : French (Making out)
2nd Base : Feel (The boobies)
3rd Base : Finger (Masturbating the other Guy/Girl)
4th Base : Fuck (To get laid!)

When a guy and girl are making out and the guy continues to try to cop a feel despite the girl moving his hand away. Eventually she gives in and allows him to reach "second base on defensive indifference."

While many know of and get to 1st, 2nd and 3rd base with a person, there is an alternate base known as 'Let-Base'.

Let-Base is an alternate base option after 2nd base where the person allows the other full access to touch any part of the body. Many find this base shameful to the one who allows it as it shows little self-respect though praise the one who achieves it.

the one after 2nd base.

When you get to home base twice in one evening, then have a threesome with said two previous people.

Group sex with your parents and your parents parents.

If one or more of the participating parents is dead and
requires exhumation before participating then 17th base
becomes: 103453rd base,103454th base,103455th base,103456th base,103457th base,103458th base,103459th base,103460th base,103461st base,103462nd base,103463rd base,103464th base,103465th base,103466th base,103457th base depending on the combination exhumed and then group
fucked with.