In a manner that follows established rules or customs. The agreement was formally signed by both parties.
In a way that is official or recognized by authority. The new policy was formally announced at the meeting.
In a manner that is ceremonious or according to traditional etiquette. He was formally dressed for the gala event.
In terms of logical form or structure rather than content; theoretically or abstractly. Formally, the problem can be reduced to a set of linear equations.