Definitions:
retain for future use

arrange for (a room, seat, ticket, etc.) to be kept for the use of a particular person

refrain from delivering (a judgement or decision) without due consideration or evidence

a supply of a commodity not needed for immediate use but available if required

a body of troops withheld from action to reinforce or protect others, or additional to the regular forces and available in an emergency

an extra player who is a possible substitute in a team

a place set aside for special use.

a lack of warmth or openness in manner or expression

(in the decoration of ceramics or textiles) an area in which the original material or background colour remains visible.