Citrus limon x Citrus sinensis
A natural cross, believed to be with sweet orange. The fruit is very thin skinned and has a sweeter than normal flesh with a unique flavour.
It has a cult following amongst some chefs.
A little tricky to grow in the UK as is dislikes our damp autumns but well worth a try for those unusual fruits.
A common garden plant in America, New Zealand and Australia.