Ash is the twerp, (Japanese: じゃりボーイ jari-boy) according to Jessie, James and Meowth. Collectively, Ash and his friends are the twerps (Japanese: じゃりの子 jari-kids; or more roughly, the brats). The term can be extended to virtually anyone who associates with Ash and isn't on Team Rocket's side. They rarely refer to the twerps by their actual names. In fact, almost always, the only times when Team Rocket calls Ash and his friends by their real names is when they are in disguises or when Ash and Team Rocket are forced to work together. This seems to be more out of disrespect than because they don't remember the names of the twerps.
When they want to refer to Ash individually, they will call him just "the twerp". However, if they want to refer to his friends individually, they will use a variation on this. For example, their nickname for Brock is "the big twerp". Also, when referring to Misty, May, or Dawn, they will sometimes refer to them as twerpette (Japanese: じゃりガール jari-girl), the feminine version of "twerp" (though it should be noted that the term "twerpette" was not mentioned in the dub until the Advanced Generation series). They also occasionally called Max the "Mini-Twerp". When May and Dawn first joined Ash on his journey, they were referred to as "the new twerp". When May returned in the Diamond & Pearl series she was referred to as the "Hoenn region twerpette". May was also referred to as "twerp with the bandanna" in May's Egg-Cellent Adventure. Paul, due to his rude and uncaring personality, is called "the rude twerp", though they have called him "the tall twerp" a couple times, possibly a reference to the fact that all of Ash's rivals are taller than him. In Strategy with a Smile! Meowth refers to Zoey as "the red-haired twerp". Barry is known as the "fining twerp" because he usually mentions to fine a person. Kenny is called "the contest twerp" in Stopped in the Name of Love! for being one of Dawn's rivals.
Additionally, when referring to themselves as Team Rocket, they will sometimes refer to the twerps as Team Twerp in comparison.
Fans sometimes use the term as "anyone who isn't a member of Team Rocket". But in the anime, Ash is the one true twerp, and all other twerps exist in relation to him.