booster car seat; booster seat
A child car seat designed for older children who have outgrown a full harness seat; uses the vehicle's seat belt. Wasei-eigo from English 'junior seat'.
wasei-eigo (Japanese-coined English), from junior seat
Since my child has grown, I replaced the child seat with a booster seat.
A booster seat can be used from around age 6.
Variant with interpunct; less common but still seen.
Wasei-eigo from English 'junior seat'. The term is not used in English for a booster car seat; in Japanese it specifically refers to a booster seat for children.