A person who has formally become a Christian out of the need for survival. He may follow the necessary requirements to appear Christian to others, such as attending church, and being baptized, but does not accept Christianity internally.
The term comes from Christian countries in Asia, such as Korea, where Christianity was forced on population under the threat of being starved to death (i.e., if you want rice, become a Christian).
It is now used for anyone who becomes a Christian for socio-economic or political reasons of survival.
Descendants of rice Christians tend to become actual Christians after a few generations. One might say that Western Christianity is the result of the Edict of Milan producing a huge number of rice Christians.
Note: I should add that though officially the Edict of Milan gave freedom to Christianity (which is good), soon thereafter you could not get any serious position within the Roman Empire if you were not a Christian.