A major triad is a chord with three notes x,y,z such that y is 4 semitones higher than x and z is 3 semitones higher than y.
Alternatively, treating notes as integers, (x,y,z) is a major triad iff (x,y,z)=(x,x+4,x+7).
Note that a major triad is the simplest form of a major chord.
Note that a major triad is composed of the first, the third, and the fifth sounds from a major scale.
Check out our definition of minor triads also explained on a virtual piano.