The greatest of our sages have offered many explanations of the word sefirot.
Some explain that the word comes from sapir, a diamond.
Without a color of its own, it reflects and refracts light or shines with an internal glow.
Others relate it to the word spur, a narrative.
The sefirot reveal God to His creatures–"the heavens tell the glory of God" (Psalms 19:2).
Alternatively, it is because we have permission to speak about the sefirot and the levels below them, but not about the levels that transcend them.
Some relate the word sefirah to s'far, or boundary, for they lie on the border between the infinite and the finite.
Others explain the word as being related to mispar, number, for the sefirot are defined by their number–that is, ten sefirot comprise one basic unit, and the sefirot bear a mathematical relationship to each other.
Other explanations are offered as well.
All of them enrich the others, although they are not all based on one shared meaning.
–Rabbi Adin Steinsaltz
From the new edition (2010) of The Thirteen Petalled Rose, p. 166, by Rabbi Adin Steinsaltz