An amateur; someone who does an activity for pleasure rather than professionally. Opposite of a professional (プロ).
彼はアマチュアの写真家だが、作品はとても上手だ。
He's an amateur photographer, but his work is very good.
アマチュアの大会に参加して経験を積みたい。
I want to take part in amateur competitions to gain experience.
趣味で演奏している人はアマチュアと呼ばれることが多い。
People who play music as a hobby are often called amateurs.