Wikipedia
A statutory instrument (SI) is the principal form in which delegated legislation is made in Great Britain.
Statutory instruments are governed by the Statutory Instruments Act 1946. They replaced statutory rules and orders, made under the Rules Publication Act 1893, in 1948.
Most delegated legislation in Great Britain is made in the form of a statutory instrument. (In Northern Ireland, delegated legislation is organised into statutory rules, rather than statutory instruments.) The advent of devolution in 1999 resulted in many powers to make statutory instruments being transferred to the Scottish and Welsh governments, and oversight to the Scottish Parliament and National Assembly for Wales. Instruments made by the Scottish Government are now classed separately as Scottish statutory instruments.
In many countries, a statutory instrument is a form of delegated or secondary legislation.