We absolutely don't want formal definitions for law. While it may seem like a good idea on first sight it would create more problems than it would solve. Lawmakers often forget some specific situations (or there is technical progress that wasn't taken into account) when writing the text of the law (or in this case the formal definition) but it's clear how their intent would be. Judges can decide those specific cases when they are on the table. Nobody can foresee everything.

There are many examples where case law differs from the exact wording of a law for a very good reason.