The Greek noun keraia literally means a little horn or a projecting point — in the context of writing, it refers to the tiny decorative stroke at the end of a letter (what we call a 'serif'), or the smallest distinguishing mark between similar Hebrew letters.
Jesus uses keraia in Matthew 5:18 to teach the absolute permanence and precision of God's word: 'not the smallest letter, not the least stroke of a pen (keraia), will by any means disappear from the Law until everything is accomplished.' This is the highest possible affirmation of Scripture's enduring authority.
Theologically, keraia speaks to the plenary inspiration of Scripture. Even the smallest detail of God's written Word carries weight and will be fulfilled. Jesus' commitment to the full authority of Scripture was absolute.