The diatonic scale has 6 5-limit triads in it. You could say that the diatonic scales is defined by having 6 5-limit triads, but that's dodgy historically. A complex note is made up of overtones or partials . These give an instrument its timbre or tone colour. With many instruments, the overtones are at special intervals, and this is the overtone series .
Overtones are also partials.
The lowest partial, which isn't an overtone, is called the fundamental . The overtones are all higher than the fundamental, hence the over bit. In older versions of this page, I got the numbering wrong on this, so there may still be wrong numberings. The second partial is an octave above the fundamental. The third and fourth partials are a perfect fifth and octave respectively above the second partial.