Arousal of the senses in a manner intended to give your partner sexual pleasure. This often involves one or both partners doing naughty things.
It almost always involves the sense of touch but any and all senses (sight, hearing, smell, (even taste) may be involved. An often forgotten ignored or not thought about bit of sexual stimulation involves conversation - flirting, naughty talk, phone sex ... and so on and so forth.
When you come down to it, sexual arousal is all in your head, which just goes to prove that the brain is the sexiest organ.