To ration is to limit the amount of something that a person or animal can have.(verb)
When you only let your dog have a cup of food a day, this is an example of a situation where you ration your dog's food.
The definition of a ration is a fixed amount of something that is provided on a regular basis, or that is the limit of how much you are permitted to have.(noun)
See ration in Webster's New World College Dictionary
Origin: MFr < ML ratio, ration < L, a reckoning: see reason
See ration in American Heritage Dictionary 4
Origin: , from Latin ratiō, ratiōn-, calculation; see ratio.
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