Answer:
[tex]\huge\boxed{\sf Nucleus}[/tex]
Explanation:
Eukaryotic Cell:
Greek Word : "Eu" means "Good" and "Karyon" means "Nut" which means that they have a Nucleus.
Prokaryotic Cell:
Greek Word : "Pro" means "Before" and "Karyon" means "Nut" which means that they lack Nucleus.
Difference:
The main difference between both is of a "Nucleus".
Hope this helped!