To me in AE:
100 = one hundred
1,000 = one thousand
1,000,000 = one million
1,000,000,000 = one billion
1,000,000,000,000 = one trillion
The terms billion, trillion, quadrillion, etc., up to nonillion, are explained by N. Chuquet, in his Triparty de la Science des Nombres (lf. 2r) printed in Bullettino di Bibliografia e di Storia delle Scienze Matematiche XIII. 593 (Roma 1880); also in the Arismetique of Ét. de la Roche, 1520. Both of these early writers explain billion, trillion, etc. as successive powers of a million, the trillion being the third power of a million, ‘a million of millions of millions’, as formerly always used in England. According to Littré, it was only in the middle of the 17th c. that the ‘erroneous’ custom was established of dividing series of figures above a million into groups of three, and calling a thousand millions a billion, and a million millions a trillion, an entire perversion of the nomenclature of Chuquet and De la Roche.
In BE it used to be that a billion was a million million, and a trillion was a billion billion.
However about 25 years ago the Chancellor of the Exchequer (= the British Finance Secretary) announced in Parliament that from then on a billion would mean a thousand million, as in the USA. I don't know if the same change occurred for a trillion.
In the UK a billion is a million million except when referring to money where the use of 1000 million to mean a billion has, I believe, become universally accepted.