1. The number of four; a collection of four things.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• cetred,
• petrad,
• tetard,
• retrad,
• tetrae,
• tetrard,
• tetradr,
• tatrad,
• tetred,
• tet rad,
• aerad,
• tetret,
• tetraid,
• tetreic,
• tetryl,
• tetrered,
• vemtrad,
• yttreia,
• heemrad,
• perad

• tetrode,
• Tetterton.

The adjective form of the word "tetrad" is "tetradic".

The plural of the "tetrad" can be the "tetrads".

The singular of the "tetrad" can be the "tetrad".