The flugelhorn is a brass instrument

Brass instrument

A brass instrument is a musical instrument whose sound is produced by sympathetic vibration of air in a tubular resonator in sympathy with the vibration of the player's lips...

 resembling a trumpet

Trumpet

The trumpet is the musical instrument with the highest register in the brass family. Trumpets are among the oldest musical instruments, dating back to at least 1500 BCE. They are played by blowing air through closed lips, producing a "buzzing" sound which starts a standing wave vibration in the air...

 but with a wider, conical bore

Bore (wind instruments)

The bore of a wind instrument is its interior chamber that defines a flow path through which air travels and is set into vibration to produce sounds. The shape of the bore has a strong influence on the instruments' timbre.-Bore shapes:...

. Some consider it to be a member of the saxhorn

Saxhorn

The saxhorn is a valved brass instrument with a conical bore and deep cup-shaped mouthpiece. The sound has a characteristic mellow quality, and blends well with other brass.-The saxhorn family:...

 family developed by Adolphe Sax

Adolphe Sax

Antoine-Joseph "Adolphe" Sax was a Belgian musical instrument designer and musician who played the flute and clarinet, and is best known for having invented the saxophone.-Biography:...

 (who also developed the saxophone

Saxophone

The saxophone is a conical-bore transposing musical instrument that is a member of the woodwind family. Saxophones are usually made of brass and played with a single-reed mouthpiece similar to that of the clarinet. The saxophone was invented by the Belgian instrument maker Adolphe Sax in 1846...

); however, other historians assert that it derives from the valve bugle

Bugle (instrument)

The bugle is one of the simplest brass instruments, having no valves or other pitch-altering devices. All pitch control is done by varying the player's embouchure, since the bugle has no other mechanism for controlling pitch. Consequently, the bugle is limited to notes within the harmonic series...

 designed by Michael Saurle (father), Munich 1832 (Royal Bavarian privilege for a "chromatic Flügelhorn" 1832), thus predating Adolphe Sax's innovative work.
The German word Flügel translates into English as "wing" or "flank".