1. A gradual decrease in the level of skill, competence, or expertise in a particular area or language
The student's declining proficiency in mathematics became evident after missing several months of classes.
A proficiência em declínio do aluno em matemática tornou-se evidente após perder vários meses de aulas.
2. A worsening ability to perform tasks or demonstrate knowledge that was previously mastered
His declining proficiency in English after moving away from an English-speaking country was noticeable.
Sua proficiência em declínio no inglês após se mudar de um país de língua inglesa era perceptível.