A dialect is a variety of a language shared by a group of speakers, defined regionally or socially. An accent is the way a given dialect is pronounced.  Dialects have other features, such as characteristic words and grammar, but strictly the term "accent" only refers to its pronunciation features.

English is pronounced in different ways around the world, according to speakers' regional and social grouping.  Every kind of spoken English has an accent – there's no such thing as English pronunciation without an accent.  

Accent "reduction" is therefore a matter of modifying one accent to make it more like another – usually, modifying a foreign accent to make it more like a native accent.