Citron tree and Citrus tree

Citron tree noun - Thorny evergreen small tree or shrub of India widely cultivated for its large lemonlike fruits that have thick warty rind.

Citrus tree and citron tree are semantically related. In some cases you can use "Citrus tree" instead a noun phrase "Citron tree".

Citrus tree noun - Any of numerous tropical usually thorny evergreen trees of the genus Citrus having leathery evergreen leaves and widely cultivated for their juicy edible fruits having leathery aromatic rinds.

Citron tree and citrus tree are semantically related. Sometimes you can replace phrase "Citron tree" with "Citrus tree", this phrases are similar.