What does STD code mean?

In the field of telecommunications in the UK, STD stands for 'subscriber trunk dialling' and is a hangover from the time when telephone users could 'phone straight through to a telephone controlled by another telephone exchange (prior to that, you had to ask the operator to put you through) The STD code is the first four (now five) digits of the full telephone number. The term STD is now a largely defunct term.