Productive prefix used in loanwords and modern compounds to mean 'partially' or 'half'. Common in words like セミプロ (semi-professional) or セミダブル (semi-double bed).
He is still a semi-professional player, but his ability is close to that of a pro.
このホテルの部屋にはセミダブルのベッドがある。
This hotel room has a semi-double bed.
semi-tight (skirt)
semi-double bed; three-quarter bed
Borrowed from English 'semi-', ultimately from Latin. Used as a prefix in modern Japanese loanwords.