Originally from Upper Burma or the Himalayan foothills of Kashmir, the ancestor of today's lemon was introduced into China around 2000 BC. After crossing the Middle East, it was adopted by the Hebrews, Greeks and then the Romans. This large fruit, with its thick, irregular skin and elongated stalk, is portrayed in Roman mosaics dating back to the 1st century. It was known as the "Medes apple", named after an ancient Iranian people close to the Persians. However, lemon cultivation ceased in Europe and the Mediterranean when the Roman Empire fell.
That was until the 8th century, when the Moors started planting lemon trees across the Mediterranean (mainly in Sicily and Andalucia). In the 10th century, they introduced "real" lemon trees into Egypt and Palestine, and crusaders brought them back into Europe.
The fruit was known as "limon", even in France, from the Italian "limone" which was itself derived from the Arab-Persian "limûn".
"Citron", the modern French word (from the Latin "citrus"), was first used in the late 14th century and gradually replaced "limon" in popular parlance.   

A Gourmet's notebook