What is Arrestable Offence?

Legal Definition
Arrestable offence is a legal term now obsolete in English law and the legal system of Northern Ireland, but still used in the legal system of the Republic of Ireland. The Criminal Law Act 1967 introduced the category to replace the ancient term felony. That Act had been superseded by the Police and Criminal Evidence Act 1984, which over the next two decades was itself significantly amended to increase police powers of arrest, relating in particular to entry, search following arrest and to custody. In England and Wales, the category "arrestable offence" ceased to exist with the advent on 1 January 2006 of the Serious Organised Crime and Police Act. In Northern Ireland, it ceased to exist with the advent of the Police and Criminal Evidence (Amendment) (Northern Ireland) Order 2007. In the Republic of Ireland, the Criminal Law Act 1997 abolished the terms felony and misdemeanour and created the term "arrestable offence" in their place.
-- Wikipedia
Legal Definition
An act with a fixed sentence or a minimum sentence. No warrant is needed if the party has or is about to commit such an offence.