I think that being stealth is the opposite of being 'out' or openly trans. Sure, you don't have to tell EVERYONE and their dog that your trans, that just isn't really necessary most of the time, but having a circle of people know you're trans, or being open about issues/amenities of life as a trans person would be, being open about it.
If you're not open about it, or you're 'stealth', to me, means that you're living a life where no one knows, or very very little people, even those close to you or even those romantically involved with you, know, and you work hard to keep it that way, keep being trans a secret. For me it was always hiding binding, denying/never bringing up 'periods', never talking in my comfort zone (always deepened my voice, always hurt at the end of the day) packing CONSTANTLY, always using the men's room, never letting ANYONE see me naked/even partially disrobed. If you're actively trying to thwart people from 'finding out' you're trans, to the point where you're keeping many things 'hidden' or 'to yourself' then I consider that stealth.
If your own partner and best friends don't know you're trans, then it's stealth, that's what I think it means.