Solar wind

Also known as a stellar wind when reaching outside the orbital boundaries of a solar system, solar wind is a continuous stream of ions (particularly protons) that radiates outwards from a sun. The solar wind is emitted where the sun's magnetic field loops out into space instead of looping back into the surface. The solar wind of a typical main sequence star can reach as far as 100 AU from the sun, effecting the entire solar system. This is why solar sail vessels are popular for inter-planetary transit and leisure cruises.

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I know of three mechanisms which can drive a stellar wind: (1) radiation, (2) Alfven waves, and (3) a magnetic rotor. Supermassive stars will shine bright enough to push the wind by radiation pressure. That works for solar type stars only when they enter the red giant stage of evolution, and get bright enough. An Alfven wave is a wave in the magnetic field, which will certainly drive charged particles, and is most likely the source for the acceleration of the solar wind. The magnetic rotor effect is based on the fact that a rotating magnetic field will transfer angular momentum to a stellar wind.

So it's not just "a" defining characteristic, but at least a few defining characteristics. Since spectral class, mass & temperature are related, it's no surprise that stellar winds will be related to spectral class too. So O, A & B stars will have faster and more massive stellar winds, in general. Wolf-rayet stars have huge stellar winds. Our own sun will probably lose about 20% of its mass to stellar winds before it settles into a long retirement as a white dwarf. A more massive star, say 5 solar masses, will probably lose over 80% of its mass to stellar winds, in its red & asymptotic giant phases. More massive, supermassive stars won't likley lose that much, but will probably lose about half their mass to stellar winds.

There is in fact a lot that is not known about stellar winds. There may be, and probably are, other mechanisms for driving winds (plain old thermal instabilities in pulsating stars?), it's a really complicated field. See Introduction to Stellar Winds, H.J.G.L.M. Lamers & J.P. Cassinelli (no typo there, Lamers has a lot of names), Cambridge university Press, 1999. It's the only full-length treatment of the topic that I know of, though there will be a chapter on winds in any book on stellar evolution or stellar atmospheres.