Plein-jeu is a compound organ stop

Organ stop

An organ stop is a component of a pipe organ that admits pressurized air to a set of organ pipes. Its name comes from the fact that stops can be used selectively by the organist; some can be "on" , while others can be "off" .The term can also refer...

 of principal tone, involving four ranks of pipes, but some examples are found with three ranks, and some rare, two ranks.
It is also sometimes known as Mixture, Cymbale, Fourniture or Scharf.

Many organ builders use the name Plein-jeu for a compound ranks

Pipe organ

The pipe organ is a musical instrument that produces sound by driving pressurized air through pipes selected via a keyboard. Because each organ pipe produces a single pitch, the pipes are provided in sets called ranks, each of which has a common timbre and volume throughout the keyboard compass...

 stop. When a single key on the organ is pressed, four notes sound, each at octave

Octave

In music, an octave is the interval between one musical pitch and another with half or double its frequency. The octave relationship is a natural phenomenon that has been referred to as the "basic miracle of music", the use of which is "common in most musical systems"...

and fifth relationships to each other. Three ranks of pipes sound three notes, and two ranks sound two notes, and so forth. As the stop progresses upward on the keyboard, the notes "break" back to the next lower octave or fifth. This stops is used to add depth, clarity, and definition to an entire organ compass, or just an ensemble of stops.

Compound principal stops are also known as mixtures, to help blend, clarify and deepen the organ tone. This is because they use a 'mixture' of pipes, which all are at octave and fifth relationships.