Abarim Publications' online Biblical Greek Dictionary
The adverb αχρι (achri), or αχρις (achris) before a vowel, expresses duration or length up onto a specified point in space or time: until or as far as. Combined with the genitive case it means during: until [the passing of] a season/day/period. This adverb occurs 50 times in the New Testament; see full concordance. It has no derivations.
The adverb μεχρι (mechri), or μεχρις (mechris) before a vowel, expresses termination of time or place, without emphasizing the process of getting there (as αχρι, achri, does). The adverb αχρι (achri) describes the whole length of a specific path, including its point of termination. Our adverb μεχρι (mechri) describes the fence around a freely roamable area. This adverb occurs 17 times; see full concordance.