Reserve in advance is a term used to describe the process of booking or preordering something before it becomes available. This can be done by booking a hotel room, reserving a table at a restaurant, or preordering a product. The crossword solutions for this term are 'BOOK' (4 letters) and 'preorder' (8 letters). Booking or preordering something in advance ensures that the desired item or service will be available when needed.