We use the term ‘register’ to refer to particular varieties or styles of speaking and writing. Registers vary because the language is used for different purposes, in different contexts and for different audiences. For example, there is a legal register, a register of advertising, registers of banking and a register of weather forecasting. We commonly recognise registers because of their specialised vocabulary but also because of particular uses of grammar. We also use the term register to refer to whether language is being used formally or informally:
[From a weather forecast register: depression is a specialised word meaning a system of weather that brings rain.]
There is a depression moving in from the Atlantic and we can expect high winds and local storms over the next few days in the north of the country.