This is a well-known false cognate, but I have to mention it here because we find it quite often in translated texts.

• In English, an alternative is a possibility, one of the possible choices.
• in French, an alternative is the choice itself.

So we should translate alternative with words such as possibilité, solution, variante (for patents) but not with alternative.

• there is no alternative -> il n’y a pas d’autre possibilité
• an alternative would be to … -> une autre solution consisterait à …

As for the adjective, alternative can be translated with autre in most cases, and not with alternative (except when talking about electricity)

• an alternative approach-> une autre approche
• alternative medicine -> médecine parallèle