What is meant by a Tabi'i?

A Tabi'i is a Muslim who met at least one Companion (Sahabi) and did not see the Prophet Muhammad, peace and blessings be upon him, after embracing Islam. This includes those who were alive during the Prophet's time but did not meet him, such as An-Najashi and Uways al-Qarani, or those who met him but did not believe during his lifetime, such as the envoy of Heraclius.