demostración
/demostɾaˈθjon/ · B2 · noun · f
presentation
Cognate of demonstration: Latin demonstrare = de + monstrare, to show
Swap English -tion for Spanish -ción and demonstration becomes demostración. Note the Spanish drops the n: demos-, not demons-. Its root monstrare (to show) also gave Spanish mostrar, so a demostración shows that something is true, which is why it also means proof.