This is the 899th most frequent Japanese word.
山口大
"山口大" (Yamaguchi Dai) likely refers to a proper noun, such as a person's name, "Dai Yamaguchi," or could abbreviate "Yamaguchi University" (山口大学). Context is needed for precise meaning.
Here '山口大' (Yamaguchi University) is used as the name of an education institution, which is common in Japan when referencing notable universities.
山口大に通っています。
I attend Yamaguchi University.
In this sentence, '山口大' functions as a personal name for an individual, illustrating how the same kanji can refer to different contexts.
彼の名前は山口大です。
His name is Yamaguchi Dai.
The term '山口大' is used again in its educational context, specifying the location being described.