I wasn’t aware that Transformers The Movie was so important.
It’s very important, because it’s an anime.
Anime is an animated film/series that is originally made in Japanese language. That thing clearly does not pass the main condition.
A Google search said:
an·i·me
ˈanəˌmā/
noun
noun: anime
a style of Japanese film and television animation, typically aimed at adults as well as children.Wikipedia says:
Anime (Japanese: アニメ) is Japanese hand-drawn or computer animation.
The Transformers along with its continuation, The Transformers: The Movie, was animated by Toei, a Japanese animation studio, and both look like any reasonably well animated anime from the 1980s.
Seems to me that it fits the criteria for being an anime pretty well.
I would say that being written for the English language originally doesn’t really hold much on the matter, especially considering that a Japanese dub was produced simultaneously, IIRC.
Wikipedia disagrees with you:
The Transformers: The Movie is a 1986 American animated science fiction action adventure film
As for the definition, there is actually no ground-truth definition for anime. The one provided by Google search is indeed very common, however it also very unclear and subjective (especially with the “style” crap). On the other hand, my definition is objective and makes a clear distinction.