What is Ihsan in Islam?

Ihsan is excellence, and the Prophet ﷺ defined it when the angel Jibril peace be upon him, known also as Gabriel, asked him, "What is 'Ihsan' (excellence)?" The Prophet ﷺ said, "Ihsan (Excellence) is to worship Allah as if you see Him. Verily, He sees you although you do not see Him." [Sahih al-Bukhari 50]

Ihsan is the third of the three fundamentals of which the religion of Islam is composed, after Islam and Iman, and the level of ihsan is Tasawwuf. Ihsan is the core of Tasawwuf; it is the perfection of yaqeen and excellence in deeds.

Tasawwuf is reaching the excellence in Iman which is the state of ihsan. Without faith and action, Iman or amal, one cannot reach the state of excellence which is ihsan.

Once a man questioned Shaykh ul Hadith Zakriya rahimahullah about Tasawwuf, and his answer was, "The beginning of Tasawwuf is the Hadith that 'deeds are dependent upon their intentions' ('Innamul a'mal o binnyat') and the epitome is that you worship Allah as you see Him (Ihsan)".