I am writing a book, quoting from Aune: “There are several instances in Revelation of this use of ἔρχεται as futurum instans…” Note that Aune uses the term “futurum instans” for Greek, not Semitic languages. Google Translate tells the term as “be instant” and tells “futurum” as “the future” and “istans” as “instant”. http://tbt.sagepub.com/content/66/2/129.abstract tells the term as “the imminent future, ‘about to’”. Perseus tells “present, immediate” for “istans” (nothing for “futurum”). If I want to give a brief definition of the term "futurum istans" to the readers of my book, could I give “the imminent future, ‘about to’”?
Futurum instans literally means "immediate/imminent future." ("Instant" comes from this word but has a different flavor in English now.)
Futurum instans seems to be a term especially prevalent in the grammar of Semitic languages. Most of the top Google results refer to Hebrew or Aramaic. It appears to be a common term for a certain Hebrew participle (see here for instance.) I have traced it to Gesenius's Hebrew Grammar:
5 The use of the participle as predicate is very frequent in noun-clauses (which, according to §140e, describe established facts and states), in which the period of time intended by the description must again (see above, d) be inferred from the context. Thus:
(c) To announce future actions or events, e.g. 1 K 22, 2 K 416 at this season when the time cometh round, אַתְּ חֹבֶ֫קֶת בֵּן thou shalt embrace a son; so after a specification of time, Gn 74, 1514, 1719, 1913, Hag 26 (but in Is 2315, where, after וְהָיָה we should rather expect a perfect consecutive, it is better to explain וְנִשְׁכַּ֫חַת, with Qimḥi, as the 3rd sing. fem. of the perfect; on the form, cf. §44f); or in relative clauses, Gn 4125, Is 55, i.e. am in the act of doing; in a deliberative question, Gn 3730; but especially often when the subject is introduced by הִנֵּה (especially also if the subject be attached as a suffix to הִנֵּה as הִנְנִי, הִנְּךָ, &c.), if it is intended to announce the event as imminent, or at least near at hand (and sure to happen), when it is called futurum instans, e.g. Gn 617, 153, 203, 2413 f., 48:21, 50:5, Ex 313, 825, 93, 3410, Jos 218, Ju 717, 933, 1 S 311, 2 K 72, Is 31, 714, 171, Jer 3010, Zc 213, 38; with a participle passive, 2 S 2021.
In the context of English, the futurum instans is sometimes used to refer to the construction "about to V", e.g. "I am about to sing."
Since Biblical Greek often applies Semiticisms, it comes as no surprise that Greek ἔρχεται (ἔρχεται means "he/she/it goes") is employed in the New Testament as a parallel of this construction. It is often used with ἰδού for this purpose.
Steven Thompson, in Apocalypse and Semitic Syntax gives more information about this:
C.F. Burney was among the first to apply the Semitic participle of futurum instans in explaining the futuristic present verbs in the NT. He explains the futuristic ἔρχομαι found often in the fourth Gospel as due to Aramaic influence of this nature, and notes that in a majority of cases where the futuristic ἔρχομαι occurs, the Peshitta represents it by a participle. (pg. 32)
Pg. 32ff is replete with details and examples of this construction in the Greek New Testament.