Manga (漫画) basically means Japanese comics. It is a very narrow term referring to comics that follow specific formats and traditions, and made for the Japanese market.
It would not, therefore, be correct to call a comic a manga, even if it was in the style commonly associated with them, if (for example) it were not created for the Japanese market.
Manga is a big deal in Japan, with a dense and intricate artistic history dating back to pre-comic traditional artforms, and a complex and storied social history. You could read ten thousand words about manga and have only scratched the surface of the phenomenon.