Any type of sexual contact or behavior that occurs without the explicit consent of the recipient.
Sexual assault is an umbrella term that refers to sexual activity that you don't want or are forced to do. Often, people think that only forced intercourse, or rape, is really sexual assault, but any sexual activity performed without permission constitutes sexual assault. This can include kissing, exhibitionism (showing someone your genitals without permission), groping, and rape. Victims of sexual assault might be convinced to do sexual acts through verbal or physical threats or by taking or being given alcohol or drugs.
An agreement between participants to engage in sexual activity.
Consent is an agreement between participants to engage in sexual activity. Without consent, all sexual activity is considered assault. There are many ways to give consent, not all of which are verbal--although verbally agreeing to different sexual activities can help you and your partner(s) establish and respect each other's boundaries.
Ultimately, consent is about communication, and needs to happen every time and every step of the way. Giving consent for one activity does not translate into consent for all activities, and giving consent once does not mean giving consent for future sexual activities. If you have had sex with someone in the past, it doesn't give them permission to have sex with you in the future.
It is important to remember that when it comes to consent, you can change your mind at any time. If you are uncomfortable, tell your partner(s) that you are no longer comfortable and want to stop. If they do not stop, they no longer have your consent and any further sexual contact is considered assault.