formalFREE/NOT IN PRISON if a prisoner or an animal is at liberty, they are no longer in prison or enclosed in a small place 〔犯人或动物〕不受拘禁的,自由的SYN free
Examples from the Corpus
at liberty• Tonight only one of the escaped prisoners remainsat liberty; the other seven are now back in Central Prison.• Walker was at liberty for many years before the police apprehended him.