Be perfect…. what???
Therefore you are to be perfect, as your heavenly Father is perfect (Mathew 5, 48)
Our common understanding (highly influenced by Aristotle) of the perfection of God is that it is impossible to achieve. And Jesus gives this impossible commandment. After all, he is a crazy fellow, who makes many unnecessary demands. Another one is, “love your enemies”.
Rather than looking for a definition, some characters of the Bible (especially from the parables) may give us some hint.