Back-formation from million, then used in billion, trillion, quadrillion, each from French, ultimately from Italian mille (“thousand”) and the augmentative suffix -one. -illion was segmented from million as if mi- was a prefix, and -illion the stem.

-illion

1. Combined with Latin prefixes for names of integers in order to form names of powers of a million.

   ‎bi- + ‎-illion → ‎billion

   ‎tri- + ‎-illion → ‎trillion

2. Added to various nonsense syllables to indicate an arbitrarily very large number.

   bajillion, gazillion

• The words formed with this suffix may mean either 👁 {\displaystyle 1,000,000^{n}}
  or 👁 {\displaystyle 1000^{n+1}}
  in different nations and languages. Hence billion can mean “one million million” or “one thousand million”. See 👁 Image
  Long and short scales on Wikipedia.Wikipedia

• -illiard

• 👁 Image
  Indefinite and fictitious numbers on Wikipedia.Wikipedia

Back-formation from million, ultimately from Italian mille (“thousand”) and the augmentative suffix -one. -illion was segmented from million as if mi- was a prefix, and -illion the stem.

• IPA^(key): /i.ljɔ̃/

-illion m (noun-forming suffix, plural -illions)

1. -illion

• -illiard