I've read in many papers something that looks an specialisation of the *block ciphers* calling them with a *lightweight* tag, but I couldn't found a *definition*.

If I understand correctly, those *lightweight block ciphers* are ciphers that due to their characteristics, are particularly interesting environments like embedded and small memory resources.

But how can be them defined formally to trace a clear line to distinguish which algorithms are, and which not, in this *lightweight block ciphers* set?