IPA: /ˌæntiˈsleɪvəri/
KK: /æntiˈsleɪvəri/
Opposed to the practice or system of owning people as slaves.
The antislavery movement fought for the rights of all individuals to be free.
Antislavery is formed from "anti-" (meaning against) and "slavery" (from Old English "sclaf", meaning the condition of being a slave). The word antislavery refers to the movement or stance against the institution of slavery.
Think of being 'against' ('anti-') the 'condition of being a slave' ('slavery') — that's what antislavery means.
No commonly confused words.