What counts as a safeword?
Is it still a safeword if you don’t tell her what it is?
There are some words that, by default, are always safewords, at least in the sense that if you say them during sex your partner should know that you want to stop. These words include “no,” “stop,” and “that hurts!” There’s a wise adage that says “no always means no.” It’s a good rule as much as it’s intended to address a sentiment that goes like this: “I didn’t think she really meant no because she looked like she enjoying it.” But the rule might go too far when it stops consenting adults from doing what they want to.
Some people get turned on by consensually enacting nonconsensual scenarios. Using a pre-agreed upon safeword to stop the scene if it begins to cross their boundaries, they are free to protest “no, stop!” as much as they like without cutting short their fun, while still ensuring they’re enjoying the scene.
Safewords are usually chosen that would not come up naturally, like “Oklahoma” or “aardvark.” Because of this, any word that is nonsensical to say during sex may successfully communicate that you want to stop. During a role-playing scene, breaking character or using your partner’s real name could communicate the same. There are also some standard safewords that should, by default at least, always be respected. These are “safeword” and “red.”
Some partners take things further by consenting not to use safewords, but if you’re asking about having a mystery word that your girlfriend guesses to try to make things stop, Rumpelstiltskin-style, that doesn’t count as a safeword.
Recommendations: Two great basic books about SM and BDSM and a beginners bondage kit.