There are multiple definitions - click on the translation that matches your context.
fundamental breach means:
substantivo
full
noun
"Violação essencial" is the term used in the United Nations Convention on Contracts for the International Sale of Goods (CISG) (in Portuguese, "Convenção das Nações Unidas sobre Contratos de Compra e Venda Internacional de Mercadorias").
the breach of an essential contract term by one party that allows the other party (=the injured party) to end the contract and sue for damages
Either party may terminate this Agreement at any time without notice in the event of a fundamental breach of the terms of this Agreement.