A difficult question to answer. We've had a lot of debate on the course about this.
Historically 'cinio' was the main meal of the day at mid-day. Welsh schools still call 'lunch' 'amser cinio' ie dinner time. This was especially strong for the Sunday mid-day meal. In my childhood in the 60's this was universally known as 'Sunday Dinner' - Cinio Dydd Sul
However as patterns of living have changed the American pattern has become more and more common. However the problem for Welsh is that there is no word for 'lunch' in the language. As a result the word 'cinio' can have multiple meanings
The simplest answer is to follow the lead of the University of Wales dictionary which refers to 'cinio' as being 'the main meal of the day'.