Visuddhimagga X-49

Vacanattho panettha oḷārikāya saññāya abhāvato sukhumāya ca bhāvato nevassa sasampayuttadhammassa jhānassa saññā nāsaññanti nevasaññānāsaññaṃ.

次说（非想非非想处的）语义：因为没有粗的想而有细妙的想，故不是与其它的相应之法共的禅的想亦非无想，故言「非想非非想」；

Ñ(X,49): The word meaning here is this: that jhāna with its associated states neither has perception nor has no perception because of the absence of gross perception and the presence of subtle perception, thus it is 'neither-perception-nor-non-perception' (n'eva-saññā-nāsaññaṃ).

Nevasaññānāsaññañca taṃ manāyatanadhammāyatanapariyāpannattā āyatanañcāti nevasaññānāsaññāyatanaṃ.

而非想非非想是属于意处及法处的处，故名「非想非非想处」。

Ñ: It is 'neither-perception-nor-non-perception' and it is a 'base' (āyatana) because it is included in the mind base (manāyatana) and the mental-object base (dhammāyatana), thus it is the 'base consisting of neither perception nor non-perception' (nevasaññānāsaññāyatana).