What exactly was the "lie" in April? The title of the series is Your Lie in April (Japanese: Shigatsu wa Kimi no Uso).

What was the lie? What was the reason for the lie?
I believe the lie was that Kaori lied about being in love with Watari. 


   On page 13 of Chapter 44. She says in her letter "This lie was that Miyazono Kaori had feelings for Watari Ryouta". She was secretly in love with Arima but didn't want to break the relationships between Arima and Tsubaki.