A common element/point — something shared among multiple items; in mathematics, an element common to sets.
この二つの問題の共通項を見つければ、解き方がわかる。
If you find the common element between these two problems, you'll know how to solve them.
クラスのみんなの共通項は音楽が好きなことだった。
The thing everyone in the class had in common was that they liked music.
集合Aと集合Bの共通項はA∩Bで表される。
The common elements of set A and set B are represented by A∩B.