What is Jaiz?
It is something that has not been forbidden religiously. This word is sometimes used instead of sahih or mubah.
There are some deeds that are sahih in terms of worldly judgments but that are not jaiz (permissible) in terms of otherworldly judgments. For instance, buying and selling things when the adhan for the Friday prayer is recited... Such an act is sound in terms of worldly judgments but it is sin in terms of religion since it entails spiritual responsibility.