A Crest is an orgasm without loss of erotic energy.
For a male a Crest is
an orgasm without ejaculation.
As summarized in the outline of Devotional Sex, one component which makes up Devotional Sex is the multi-orgasmic man technique taken from the Tao. This is the subject of Chapter 3 of this site.
Practicing the multi-orgasmic man technique a Knight
is able to orgasm without ejaculating.
It is when a male ejaculates that he looses all his erotic energy. This `normal´ orgasm with ejaculation is called a Climax.
After a Crest a man still has all his erotic energy. So after a Crest a man remains erect and keen for more
sexual activity. This can include another Crest, and then another.
Hence he is multi-orgasmic.
Having
enjoyed a Crest or two during sex, it is relatively easy for a Knight
to not feel frustrated when sexual activity ends without him having
ejaculated.
For a female a Crest is
an orgasm after which she still wants to continue with sex.
For a female an orgasm is a Crest or a Climax depending on how she feels after her orgasm.
If she still feels enough erotic energy to want to continue sexual activity then her orgasm is called a Crest.
If
just after her orgasm she looses most of her erotic energy, and thus her
desire for more sexual activity, her orgasm is a Climax.
Note
that wanting to end sex because of loss of erotic energy after an
orgasm is like wanting to stop eating because you feel full. It is the
body which is telling the brain what to do, and the brain just responds
to the bodies feelings.
Of course there will be many times when
a woman looses some but not all of her erotic energy. In this case it
is her mind rather than her body which decides what to do next. If she
chooses to continue with more sexual activity then her orgasm would be
called a Crest, and if she decides to end sex then it would be a Climax.

The reasons I chose the word Crest is given in Chapter 1 here.
The other new words used in Devotional Sex are listed in Chapter 1 here.