Seal in is a phrase used to describe the action of enclosing or confining something within a particular space or container. There are a number of synonyms that can be used to convey this meaning, including enclose, contain, trap, encircle, fence off, isolate, and hold in. Each of these words has slightly different connotations and can be used in different contexts to describe different actions. For example, while "enclose" may suggest placing something inside a box or container, "fence off" may suggest creating a physical barrier around an area to prevent entry. Ultimately, the choice of synonym depends on the specific context and desired meaning of the phrase "seal in".