An antipassive voice construction is the coded alternant in an °antipassive voice alternation.
The definition is from Haspelmath (2027). It is completely parallel to the definition of "passive (voice construction)".
antipassive construction (CXN) = any construction that is used for a P participant that is less salient than it usually is in the basic voice construction. Example: West Greenlandic inun-nik tuqut-si-vuq ‘He killed people,’ with an oblique flag -nik for the argu- ment phrase expressing P and an overtly coded verb form with the suffix -si, is an instance of the antipassive construction. (Section 8.4)