Asterisms

In astronomy, an asterism is a distinctive pattern of stars that is not a constellation. It can be the most distinguishing feature of a single constellation, or it can be spread out over a few constellations. The Big Dipper is an asterism, not a constellation, and is the most famous asterism in the sky.

Here are some well-known asterisms:

Big Dipper in Ursa Major
Little Dipper in Ursa Minor
Summer Triangle
Northern Cross in Cygnus
Teapot in Sagittarius
Keystone in Hercules
W in Cassiopeia
Great Square in Pegasus
Sickle in Leo
Orion's Belt

References:

Wikipedia
List of Asterisms