The trombone is a musical instrument in the brass family. As with all brass instruments, sound is produced when the player's lips vibrate inside a mouthpiece, causing the air column inside the instrument to vibrate. Nearly all trombones use a telescoping slide mechanism to alter the pitch instead of the valves used by other brass instruments. The valve trombone is an exception, using three valves similar to those on a trumpet, and the superbone has valves and a slide.

The word trombone derives from Italian tromba (trumpet) and -one (a suffix meaning 'large'), so the name means 'large trumpet'. The trombone has a predominantly cylindrical bore like the trumpet, in contrast to the more conical brass instruments like the cornet, the flugelhorn, the baritone, and the euphonium. The two most frequently encountered variants are the tenor trombone and bass trombone; when the word trombone is mentioned alone; it is mostly taken to mean the tenor model. These are treated as non-transposing instruments, reading at concert pitch in bass clef, with higher notes sometimes being notated in tenor clef. They are pitched in B♭, an octave below the B♭ trumpet and an octave above the B♭ contrabass tuba. The once common E♭ alto trombone became less common as improvements in technique extended the upper range of the tenor, but it is regaining popularity for its lighter sonority. In British brass-band music the tenor trombone is treated as a B♭ transposing instrument, written in treble clef, and the alto trombone is written at concert pitch, usually in alto clef.

A person who plays the trombone is called a trombonist or trombone player.

Trombone comes from the Italian word tromba (trumpet) plus the suffix -one (large), meaning 'large trumpet'.

During the Renaissance, the equivalent English term was sackbut. The word first appears in court records in 1495 as shakbusshe. Shakbusshe is similar to sacabuche, attested in Spain as early as 1478. The French equivalent saqueboute appears in 1466.

The German Posaune long predates the invention of the slide and could refer to a natural trumpet as late as the early fifteenth century.

The sackbut appeared in the 15th century and was used extensively across Europe, declining in most places by the mid to late 17th century. It was used in outdoor events, in concert, and in liturgical settings. Its principal role was as the contratenor part in a dance band. It was also used, along with shawms, in bands sponsored by towns and courts. Trumpeters and trombonists were employed in German city-states to stand watch in the city towers and herald the arrival of important people to the city, an activity that signified wealth and strength in 16th-century German cities. These heralding trombonists were often viewed separately from the more skilled trombonists who played in groups such as the alta capella wind ensembles and the first orchestral ensembles, which performed in religious settings such as St Mark's Basilica in Venice in the early 17th century. The 17th-century trombone had slightly smaller dimensions than a modern trombone, with a bell that was more conical and less flared. Modern period performers use the term "sackbut" to distinguish this earlier version of the trombone from the modern instrument.

Composers who wrote for trombone during this period include Claudio Monteverdi, Heinrich Schütz, Giovanni Gabrieli and his uncle Andrea Gabrieli. The trombone doubled voice parts in sacred works, but there are also solo pieces written for trombone in the early 17th century.