The word 'Temperament' indicates 'building up of musical notes'.
There can be many ways of such build-ups from Shadja to Tara Shadja. Hence, there are Equitempered scales with 12, 19, 22, 24, 31, 53, and 72 notes.
When we use the word 'tempered' in the context of Harmonium, it means a short form of '12-Tone-Equi-Tempered (ET) scale' popular in the world for the last 200 years and which is tuned in Pianos, Organs, Accordions and all traditional Harmoniums.
The 12-Tone ET scale permits an 'equal' amount of 'mis-tuning' in each note, without having to provide 12 pitches per Octave, enabling a player to play from any key taken as the fundamental tone or Shadja.
The difference in pitches thus distributed equally is called as 'dividing roughness equally in 12 notes', and is termed as 'Wolf'. Wolf has to be set right, otherwise the instrument will howl out of tune.
This scale is therefore criticized even in the West by eminent musicians as it does not have 'natural' notes, and to that extent it is 'tampered'.
For eg.,
1. Yehudi Menuhin mentions in his autobiography entitled 'An unfinished Journey' that:
"(The) West had to invent the tempered scale, each note adjusted up or down from it's true center, to reconcile to different keys. I can't pretend to regret a development that has fed my whole musical life, but equally, it is difficult to deny that tempered scale corrupts our western ears..."
- YEHUDI MENUHIN
Brian Godden from Santa Rosea, USA, another Western scholar and a student of Ustad Ali Akbar Khan Saheb, mentions on his website that:
"For the past 200 years, the ears, minds and hearts of Westerners have been saturated with ET scale consisting of 12 equally spaced 'Out-of-tune' notes based on a ratio of 1.0594631. They hear ET as 'in tune'..."
Bob Fink, yet another Western scholar mentions on his website that:
"Most ears are forgiving. Millions of people have adopted to the tempered 'dissonance' of piano tunings. They hear it with 'Self-delusional' mental adjustments..."