In American English, the sexual bases derive from baseball terminology. There are three in total, as well as the coveted “homerun”. These euphemisms are used to describe the various levels of physical intimacy that a couple can experience, usually during the progression of a romantic relationship.
First base means mouth-to-mouth kissing, especially French. This is pretty standard fare among teens and young adults, and considered to be the baseline for any type of sexual contact.
Second base involves making out while having direct skin-to-skin contact. This means fondling of the breasts, or slipping hands under clothes and groping. It can also refer to manual stimulation, though most people today would consider second base to be “making out”.
Third base is a heavier form of making out where the couple will touch below the waist while kissing, and oral sex also usually falls under this category as well.
A home run is considered the ultimate level when it comes to American sex slang, since it’s the point in which a couple engages in sexual intercorse.
In addition to the three bases and a home run, there are some other baseball terms used such as “strikeout”, which means to be turned down while trying to initiate foreplay or any other sexual activity”. Some terms are also used to describe a person’s sexual orientation, though these are now considered outdated and can be offensive. You may also hear of young people “running the bases”, which refers to teens and young adults exploring their sexuality and fooling around in multiple relationships.