I see three specific meanings that we generally apply to the word, “masturbation.” One contextual, one an objective action, and one euphemism.

And of course there are many more. This is, after all, the most common sex our species practices, so there are myriad applications and twists on the word, but I have a theory that a good 90% of us think of these three general meanings when we refer to masturbation. Here’s my grand theory:

1. Solitary sexual stimulation in general, particularly of the genitals. The operative word is “solitary.” This definition of masturbation assumes that one is always alone with one’s own pleasure (or shame), specifically giving to oneself and receiving from oneself. No other person is physically involved in the act, although others may be virtually involved through imagination (fantasy), auditory (phone sex), visual (porn) or other surrogate means (voyeurism, fetish objects, etc.). This can include sex toys or any number of enhancements, and may or may not culminate in orgasm. To qualify as masturbation, it must be a solitary practice.
2. Stroking genitals with the hand. The focus is on the specific activity of genital stimulation without penetration, chiefly with the use of our most readily available manipulative tool, the hands and fingers. This idea of masturbation defines itself by what’s being done rather than who is doing it or what context it happens in. From this perspective, a partner or group of any size may masturbate together or masturbate each other, also employing the use of toys if desired, as long as no part of one body goes inside another. The moment of oral or anal contact to genitals is where the masturbation ends and penetrative sex begins. 
3. Wasting time. “Masturbation” works as a euphemism for spending time doing anything that is considered useless by someone. Unfortunately, this definition carries with it the stigma of sexual self-stimulation as “substitute sex” which is therefore not valid, not appropriate, not “good,” not “sex.” While it may relate to something that is entirely nonsexual (i.e. “mental masturbation”) it always assumes that intercourse is superior to masturbation, that masturbation is, literally, a waste of time. This is true if the only measure of value in a sex act is procreation, in which case everything less than penis-ejaculating-in-vagina intercourse is “less than” sex.

Whatever we mean when we talk about masturbating, I think it’s enormously valuable to think about the words we use and be intentional about them. What do we want masturbation to mean? How do we actually experience the act of masturbating in different situations? While we jack off alone, are we thinking it’s a waste of time, or does that thought ever occur after orgasm? If we’re masturbating with a friend or lover, or manually pleasuring them, is that a waste of time? Is talking about it a waste of time?

We all have a choice to express ourselves any way we please. I propose that it is worthwhile to commit to speaking of masturbation positively, to claim it as fundamentally healthy and good for a vast array of reasons. Our brains often follow our mouths.