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Legal definition of the gift is - voluntary transfer of property made without Consideration - thus the transfer for which no value is received or expected to be received in return.
An engagement ring is a gift in contemplation of marriage - so it is not a regular gift "without consideration". That gift is conditioned upon the marriage actually taking place. If the marriage takes place, the condition is satisfied, and the spouse is permitted to keep the ring. In case of broken engagement the ring must be returned.
The same is true for all other condition gifts. The person may sue you if you refuse to return the ring.
However, some courts may still consider the ring as a regular gift if it was made on birthday, Christmas, etc.
Please see for reference - http://family.findlaw.com/marriage/marriage-more/broken-engagement.html