score:13
I must preface this with the fact that I'm not an expert on this subject (meaning the Catechism, not masturbation).
However, the text itself seems to show very clearly that it's the intention that matters ("in order to derive sexual pleasure."). Clearly, ejaculation isn't required, merely the act of beginning (or continuing) something that is pleasurable.
It seems clear that "deliberate stimulation" is the sin. Meaning that it has to be deliberate and it has to be stimulating. If you unintentionally stimulate yourself, it's not a sin. Also, if you intentionally do something but it's not stimulating, that's also not a sin (that would just be weird).
The definition seems obvious to me.
Having said this, the culminating sentence of this section states:
To form an equitable judgment about the subjects' moral responsibility and to guide pastoral action, one must take into account the affective immaturity, force of acquired habit, conditions of anxiety, or other psychological or social factors that lessen or even extenuate moral culpability.
So while the definition is obvious and clear (per the question), there is some leniency given to this situation.
Upvote:3
Here's my contribution to Wikipedia on this subject:
St. Thomas Aquinas, the most prominent Doctor of the Roman Catholic Church, writes that masturbation is an "unnatural vice," which is a species of lust like be*t**lity, sodomy, and pederasty, and that "by procuring pollution [i.e., ejaculation apart from intercourse], without any copulation, for the sake of venereal pleasure β¦ pertains to the sin of 'uncleanness' which some call 'effeminacy' [Latin: mollitiem, lit. 'softness, unmanliness']." [Summa Theologica II-II q. 154 a. 11 co.]