The Vietnamese word "biến cách" is a noun that refers to "noun declension," which is a grammatical term used to describe the way nouns change form based on their grammatical role in a sentence (such as subject, object, etc.).
In Vietnamese, nouns do not change form as they do in some other languages (like Latin or Russian). However, "biến cách" helps us understand how nouns can be used in different contexts and how we might modify them with other words or phrases to convey specific meanings or relationships.