to grow or increase; also, with God as the one acting, to make something grow
At its heart this word describes living, organic growth: something small becoming larger, fuller, or more mature over time. It is the word for a child growing up, a seed becoming a plant, a people becoming numerous, a movement spreading, and a believer maturing in faith. The picture is always of gradual, natural development rather than a sudden leap.
The same word works two ways. Most often it is used of the thing that grows (the child grows, the plant grows, the church grows). But it can also be used of the one who causes the growth, and in the New Testament that one is almost always God: Paul plants and Apollos waters, but God is the one who makes it grow. Both uses share the single idea of increase; the difference is only whether the focus falls on the growing thing or on the God who gives the growth.
Because the growth is organic, the New Testament writers reach for it as a picture of the life of faith. The gospel spreads like a living thing, the church is a body or a temple that grows toward completion, and a Christian is meant to keep maturing rather than stay an infant. In every case the word carries the quiet assurance that healthy life increases over time.