This is the 1109th most frequent Japanese word.
電子
"電子" means "electron" in English.
電子 (den-shi) in this case refers to 'electronic', and '電子メール' means 'electronic mail' or 'email'.
電子メールを送ります。
I send an email.
Here, 電子 (den-shi) is used to mean 'electron', a subatomic particle in physics.
電子の性質を研究しています。
I am researching the properties of electrons.
In this case, 電子 (den-shi) indicates 'electronic' as part of '電子辞書', which means 'electronic dictionary'.