What is a horn in the jazz world?

An instrument that has three roles in a group: 1. melody, 2. accompaniment, and 3. soloing. The horn does not necessarily use every role in each tune.

Melody

The horn plays the melody of the tune whether solo or in unison with others. Sometimes a horn will only play a section of the melody.

Accompaniment

Accompaniment can take multiple forms. One way is when there is a different musician playing or singing the melody, the horn supports this melody by adding in some countermelodies or phrases that do not overpower the melody. This requires a horn player to be very skilled to know when to play and when to give the other musicians space.

Another type of accompaniment, sometimes called backgrounds, refers to horn players playing an arranged, or made up, melody during another player's solo. It can happen any amount of times and should not be too busy to interfere with the soloists solo. Backgrounds take into account the range and volume of the instrument playing the solo.

Soloing

A horn plays freely over sections of the music often accompanied by the rhythm section. Usually other horn players do not play during the solo. Soloing can be in the form of trading whereby one instrument play a few measures and a different instrument plays a few measures, continuing to trade off soloing.

Is cello a horn? When did cello join the jazz world?

There are photos from early 1900s of cello in jazz bands. Jazz began as music you could play with any instrument you had lying around, so why not cello?

Sources:

1. Nat Shilkret Orchestra image: http://www.thepeoplehistory.com/20smusic.html

2. W.C. Handy's Orchestra image: https://img.discogs.com/KrF1Olz0PvbuHzNXV2_KVx3nWE8=/582x359/smart/filters:strip_icc():format(jpeg):mode_rgb():quality(90)/discogs-images/A-3101048-1557302390-9403.jpeg.jpg