In music, a pentatonic scale is a scale with five notes in each octave. Pentatonic scales are very common and are found in music from all over the world, most notably in folk music. It is the scale from which Pentatonix got their name, having five members. The X was added to make it seem cooler.
Any scale using five notes is "pentatonic" ("penta" is Greek for "five"). However, the pentatonic scale which is used in most Western music uses notes which do not have any semitones. A pentatonic scale starting on C will use the notes C, D, E, G, A (the 1st, 2nd, 3rd, 5th and 6th notes of a major scale). An easy way to find such a pentatonic scale is by using all the black notes of a keyboard.
Many folksongs are pentatonic, or nearly pentatonic. Well-known songs such as "Land of the Silver Birch" or "Auld Lang Syne" are pentatonic tunes.