The Arabic word Wasat (or Wasatiyyah) refers to the Islamic concept of moderation, described as “best, middle, centred, balanced.” The concept is described in multiple passages in the Quran and Ḥadīth.
Wasat describes the need to avoid extremes. This is my favourite quotation:
The religion (of Islam) is easy, and whoever makes the religion a rigour, it will overpower him. So, follow a middle course (in worship); if you can’t do this, do something near to it and give glad tidings and seek help (of Allah) at morn and at dusk and some part of night.