Alternatives générales :
* Concept fondamental
* Principe de base
* Idée clé
* Thème central
* Théorie majeure
Exemples spécifiques :
* Loi de conservation de la masse : This is a fundamental concept in chemistry that states matter cannot be created or destroyed in chemical reactions.
* Loi périodique : This is a key idea that explains the recurring properties of elements based on their atomic number.
* Théorie moléculaire cinétique : This is a major theory that describes the behavior of gases and explains their properties.
Ultimately, the best alternative depends on the specific big idea you are referring to and the context of your discussion.