Triangulum is one of the smallest constellations in the northern hemisphere. Its name means "triangle" in Latin. The brightest stars in Triangulum form a long triangle. The constellation was introduced by the Greek astronomer Ptolemy in the 2nd century.
The Greeks knew the constellation as Deltoton, a name they called it because it resembled the capital letter delta. Eratosthenes claimed the constellation represented the delta of the river Nile. In the 17th century, the Polish astronomer Johannes Hevelius introduced Triangulum Minus, a smaller triangle marked by three stars lying next to Triangulum, but this name is no longer in use. The Chinese knew 
delta and 
gamma Trianguli as Tianda Jiangjun, the general and his soldiers. To the Babylonians, the stars of Triangulum together with 
gamma Andromedae represented the Plough, a constellation documented as early as 1,000 BC.
The constellation Triangulum occupies an area of 132 square degrees and does not contain any stars with known planets. It can be seen at latitudes between +90° and -60° and is best visible at 9 p.m. during the month of December.
The brightest star in Triangulum is 
beta Trianguli, sometimes also known as Deltotum. It is a white giant, approximately 124 light-years distant.
alpha Trianguli, the second brightest star, is a spectroscopic binary lying about 64.1 light-years from Earth. It is also known as Mothallah or Rasal Muthallah and Caput Trianguli, both meaning "the head of the triangle." It is a yellow-white subgiant with a companion star lying only six million kilometers away.
gamma Trianguli is a white dwarf approximately 118 light-years distant.
delta Trianguli is another spectroscopic binary star, about 35 light-years distant. It consists of a yellow dwarf and an orange dwarf.
The most notable feature in Triangulum is the Triangulum Galaxy, or Messier 33 (NGC 598), a deep sky object that can be observed in binoculars. The Triangulum Galaxy is a member of the Local Group of galaxies. It is a spiral galaxy some three million light-years away from Earth. It is sometimes also called the Pinwheel Galaxy, a name usually associated with Messier 101, a spiral galaxy in the constellation Ursa Major
. The Triangulum Galaxy is the third largest galaxy in the Local Group, with a diameter spanning approximately 50,000 light-years.
Triangulum also contains several other galaxies, but none of them are brighter than magnitude 11. The largest one is NGC 925, a barred spiral galaxy approximately 45 million light-years distant.
Triangulum belongs to the Perseus family of constellations, along with Cassiopeia
Constellations directly bordering Triangulum are Andromeda