Plethyno (πληθύνω) means to multiply, to increase greatly, or to cause to grow in number. Related to plethos (G4128, multitude), it describes the action of becoming numerous or abundant.
Plethyno echoes the creation mandate of Genesis 1:28 — 'be fruitful and multiply.' Luke uses it repeatedly for the church's growth: 'the word of God continued to increase' (Acts 6:7), 'the word of the Lord continued to increase and spread' (Acts 12:24). Peter opens both epistles with 'Grace and peace be multiplied to you' — praying that God's grace would multiply exponentially. God is in the business of multiplication. What He blesses does not merely grow linearly — it multiplies.