I - ISOFIX
ISOFIX - Fitting a child seat can be fiddly, and if mistakes are made, the seat may not protect your child as it should. The ISOFIX system aims to solve this problem by providing mounting points fixed to the body of the car. This makes it simple to 'plug' the seat into place and it will be held very firmly.
The name stands for International Standards Organisation FIX, and the original idea was that all cars would have uniform mounting points; however, this has not yet been achieved.
Many car makers build their own ISOFIX seats, which are not approved for use in other makes of car. For example, What Car? found that a Toyota ISOFIX seat would not fit in a Honda Civic.