This is the 3294th most frequent Japanese word.
志願
"志願" means "aspiration" or "voluntary application."
In this sentence, '志願' is used to indicate the action of formally applying or expressing a wish to enroll in an institution.
彼は名門大学への志願を決意した。
He decided to apply to a prestigious university.
Here, '志願者' (志願 + 者) refers to a person who willingly applies or volunteers for an activity.
彼女はボランティア活動への志願者として名乗りを上げた。
She volunteered as an applicant for the charity activity.
In this case, '志願' indicates the voluntary action of applying or enlisting oneself into military service.