Personal Defense Weapon (PDW) is a lightweight easy to use handgun. While it is larger than a common Glock pistol it will be smaller than a full sized AR-15.  They are most commonly chambered in 9mm but can come in 5.56, 45 ACP, and 300 BLK.   The H&K MP5 is common thought of at the first PDW but there are other German WWII guns that would predate the MP5.  Recently companys like Daniel Defense, Noveske, and Sig Sauer have put their own "spin" on this popular category of personal defense weapon.