From: Upstate New York USA
As Apache85 alluded to (and he knows FAR more than I), there is no definition of "stealth." Stealth is mainly a media term that's been adopted into daily usage. There is no specific characteristic that defines stealth, its just sort of a relative reference that means "better" than other platforms. As such, you won't find the term incorporated into the database entries, along with other terms like "invincible", "doomsday weapon," and the more broad,"kickass."
If you want knowledge about these types of platforms, start with what you already did - some internet research on the platforms. For their in-game performance, you can check the database. All that gobbledygook at the bottom of the database entries that talks about decibels (Db) shows each platform's cross-section (radar or sonar profile). You can compare the entries to each other to see how stealthy one platform is compared to another. This is more technical than what I need for the game, but if you want to dive in, that's a good place to start.
"It's all according to how your boogaloo situation stands, you understand."
Formerly known as Colonel Mustard, before I got Slitherine Syndrome.