a spring: the living source from which water flows, and by extension the source or origin from which anything (life, blood, tears, cleansing) pours out.
At its root this word names a place where water comes up out of the ground: a spring, the living source that feeds a stream. From there it reaches naturally for anything that pours out or gives rise to something else, so the same word can name the bodily source of a woman's flow of blood, the wellspring of a person's life and wisdom, or God himself as the source of living water.
What ties every use together is the idea of a source, the point something flows from, rather than a pool that merely holds it. That is why the prophets set it against a cracked, stagnant cistern (בּוֹר): a spring keeps giving because the water is alive and moving, while a stored tank runs dry. It overlaps closely with the ordinary word for a spring (מַעְיָן), but this word leans more toward the source or origin of a thing, and it is the word the wisdom writers reach for when they speak of a "fountain of life."
In the wisdom writings the picture becomes a favorite way to praise what sustains and refreshes: the teaching of the wise, the fear of the LORD, and understanding are each called a fountain of life. In the prophets the image turns pointed. To abandon God is to forsake the fountain of living water and dig leaky tanks instead; when God brings cleansing, he opens a fountain to wash sin away.