As sado was theorized in the Edo period, the characters '懐石' (kaiseki) which were related to onjaku (a warm stone kept in the pocket of Zen monks to keep hunger away) in the Zen sect started to be used.
江戸時代になって茶道が理論化されるに伴い、禅宗の温石（おんじゃく）に通じる「懐石」の文字が当てられるようになった。