Why the lanthanide series is called lanthenide? Reason
Based on the name of the first element in the series, Lanthanum.
The element Lanthanum was discovered first between those in the series, and so when the whole group of elements with similar properties was found, it was named after Lanthanum.
Lanthanum itself was named as it is based on Greek λανθανειν (lanthanein) meaning "to lie hidden", since it was very rare and found "hidden" in other minerals.
(source: Wikipedia )