Mat 13:31 The kingdom of heaven is like to a grain of mustard seed, which a man took, and sowed in his field:

Notice that this parable and the next come between the Parable of the Good and Bad Seeds and Christ's explanation of that parable that we write about here. There is a reason for this. Both this Parable of the Mustard Seed and the following Parable of the Yeast and the Dough explain Christ's symbols in greater detail so that the explanation of the Parable of the Seeds can be understood as something much more than an "end of the world" description of judgment.

"Grain" is not from the Greek word for seed (sperma) used in the next verse, but from kokkos, which means "a grain" and "a seed," but interestingly is used as a metaphor for a "grain of sense," which fits directly into the meaning here.

"Mustard seed," though translated as seed, is sinapi, which means simply "mustard."

The other terms here, speiro for "sowed" and agros for "field," we have discussed before.