Any play that has two meanings, regarding tactics or strategy or both. For example double atari, or a play that defends your group while attacking your opponent, or a good ladder breaker that has some effect. Another tactical example is the double threat ladder-maker.
- Karami is a technical term for double-purpose attacking plays that cut off and separate the opponent's weak groups, known also as splitting attacks.
Much of basic strategy aims at setting up double-purpose plays, or denying them to your opponent. See for example immediate life. Clearly they are in general useful, and a sign that you are co-ordinating different objectives. Sometimes they are also cases of the miai concept, but certainly not always.
However, you should be wary of reaching too hard for dual-purpose plays - finding moves that try to do two things at once, but do neither of them well. A true double purpose move doesn't compromise on either of its goals.