Pneuma means spirit, breath, or wind — the Greek counterpart to the Hebrew ruach. The same single word carries all three senses, and Jesus plays on them with Nicodemus: the wind (pneuma) blows where it wishes, and so is everyone born of the Spirit (pneuma). The Spirit is as real, and as free, as the wind you feel but cannot control.
Pneuma is the New Testament's word for the Holy Spirit — the breath of God who gives new life, dwells in believers, and produces his fruit in them. It reminds us that spiritual life is not self-generated any more than we can manufacture wind; it is the gift of God's own breath, life from beyond ourselves.
Pneuma in Scripture
The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth: so is every one that is born of the Spirit.