The Chesterfield coat is a long, tailored overcoat

Overcoat

An overcoat is a type of long coat intended to be worn as the outermost garment. Overcoats usually extend below the knee, but are sometimes mistakenly referred to as topcoats, which are short coats that end at or above the knees. Topcoats and overcoats together are known as outercoats...

. It arose along with the lounge suit as an alternative to the highly shaped coats it replaced, such as the frock overcoat with its heavy waist suppression using a waist seam. The Chesterfield has no horizontal seam or sidebodies, but can still be somewhat shaped using the side seams and darts. It can be single- or double-breasted, and has been popular in a wide variety of fabrics, typically heavier weight tweeds, or charcoal and navy, and even the camel hair classic. It has often been made with a velvet collar. These variations make it extremely versatile, so it can be worn with a city suit or even semi-formal

Semi-formal

In Western clothing semi-formal is a grouping of dress codes, indicating the sort of clothes worn to events with a level of protocol between informal and formal...

dress, as well as casual sports jackets. It was a staple of smartly dressed men's wardrobes from the 1920s to 1960s, and is still very commonly worn.