a seed: a single kernel or grain of a plant
In the New Testament this word always means one thing: a seed, a single kernel of a plant. Outside the Bible the same Greek word could mean a lot more, including the crushed insect that produced the most expensive scarlet dye of the ancient world, but the New Testament writers never use it that way. When they wanted the color scarlet they reached for a related word instead. Here the word is only ever a seed, and a qualifying word usually tells you which crop: a seed of mustard, a grain of wheat.
The two senses below are the same plain object seen from two angles. In the first, the seed stands for how small something is: Jesus points to the mustard seed, proverbially the tiniest thing a person plants, to describe both the hidden beginnings of God's kingdom and the small measure of faith that is still enough. In the second, the seed stands for the way a kernel must be buried and seem to die before it can grow into something far greater: Jesus applies this to his own death, and Paul applies it to the burial and resurrection of the body.
Because the smallness and the death-and-life are two natural properties of one ordinary seed, this is one word with a single meaning, used in two vivid ways, rather than two different words that happen to look alike.