Definitions

1. noun

A hypothetical class of elements predicted by Mendeleyev’s periodic table, now known as the lanthanides and actinides.

“The discovery of the lanthanides and the actinides filled the gap in Mendeleyev’s periodic table that was previously referred to as the actinoida.”

2. adjective

Relating to or characteristic of the actinides.

“The properties of the actinoida elements are quite unique and have been the subject of much scientific study.”