Aceflexible or Flexible Asexual is a category for a type of asexual who is mostly asexual but either occasionally can experience having feelings or desires similar to but not the same as sexual attraction or for those who rarely feel sexual attraction, but for this to be so little and so rare that one would still consider themself asexual despite the sexual fluidity, something that distinguishes an aceflexible from a greysexuaI. An example of this is an asexual experimenting with sexual activities, but not feeling any attraction or desire and for reasons unrelated to attraction (cupiosexuality, libido etc). Another example would be an asexual who associates libido or arousal with sensual attraction, but does not experience sexual attraction or an asexual who felt what can be described as sexual attraction, but not feeling like they fit the sexual category because it was only visual attraction (Miransexual) and thus does not change their orientation. Similar to greysexual, it is very broad and can be both an umbrella term or an identity.
An alternative definition, is one where the term "aceflexible" has the same meaning as dark greysexual .
History
Aceflexible was first coined in AVEN on February 9th 2013 in a AVEN thread discussing it made by the coiner. It was coined by AVEN user R_1. Aceflexible currently faces issues similar to semisexual to where it's not fully defined and hadn't reached a full understanding. However, it was also controversial during the time of it's creation and people had a lot more of a narrow understanding on sexuality than we do presently today. New identities such as acefluid and aceflux as well as expansion in greysexuality, show that asexuality is a more vast spectrum, and the aceflexible label perhaps could fill a gap for asexuals who do not fit in the grey sexual spectrum.
Aceflexibility as a concept was also mentioned by users on AVEN who didn't like the new redefinition of Asexuality as "little to no sexual attraction" instead of "no sexual attraction" and that Aceflexibility would make more sense to describe people whose attraction is so little and so rare as to seem insignificant.
Resources
https://www.asexuality.org/en/topic/83926-ace-flexible-label/