Someone who is purposefully posting on a forum/messageboard/site sith the sole aim to irritate the regular members.

Contrary to popular belief there is much more to internet trolling than posting "OMG UR ALL SO GHEYYYY!!!1" "OMFG TRIVIUM SUX ASS U FAGS LOL" and the like. People who post such things will usually be one-post wonders and get banned, however the more experienced troll will attempt to cause some sort of havoc under the admins radar.

Common examples of trolling:
The long-term troll:
These guys don't mind waiting around for the fun to begin. They may pretend to be an average n00b, and give themselves the stereotyical interest(s) of the average member of said forum, then once having a few posts they will gradually try to spread dissent. This type may even pretend to be pally to the admin and get their msn (and try to crack their account password of course) whilst causing general disruption. May act under other proxies and slate these fake trolls on msn to the aforementioned gullible admin. These people, to be frank, usually have no life and love bringing message boards to their knees by putting considerable time into their carnage. If they crack the admins password the board is doomed.

The sophisticated troll:
This type will usually have some sort of distaste for whatever the said message board is discussing and try to cause annoyance as opposed to destructiveness. They will parody the members and see how much they can get over their heads. Subtle pisstaking being their route. They'll usually get banned in the end but will annoy an entire community using this drawn out approach.

The faker troll:
A clever troll, the main aim of the game is to, ironically, act really REALLY stupid, this trolls aim is to argue with members incoherantly, usually using text speak. This is a ploy, the troll only needs to spend 10 seconds writing the babble when the member wastes 10 minutes writing a much more wordy reply. The troll will reply with a stupid rebuff making the member post as many long replies as possible. Using the members weight against them and wasting their forum time... If they are good then their act will be believable and they probably won't get banned unless the admin is strict on txt tlk.

The rank and file spammer:
These are the first to pop into peoples minds, they are similar to the examples stated in the first paragraph. No previous experience necessary, this lot will swarm in, again, usually under proxies to maximise the destructiveness, and spam the place to hell. Only causes mild irritation if not in large numbers.

Join any popular forum for a year and I guarantee you'll see all of these.
(let's assume it's a metal board)

Faker troll (expecting to cause irritation):
"Hi im nu here u all seem kl i am rly metal rawrrr i leik slipnot and fall out boys!!!1 they r so metal! see u al on da forum xxx"

Rank and File spammer:
"LOOOOL U METAL PPL R SO GHEY WITH UR SHOUTY MUSIC WRIST SLITTING ANIMAL FUCKING SATAN SHIIIT"

by PsychoFox February 12, 2008

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Troll-
An angry, bitter dwarf-sized creature that is found hanging out and hiding under bridges. If you happen upon a Troll while crossing a bridge you may encounter them threatening you as well as being offensive and quite obnoxious in their behavior.
"The city bridge Troll threatened my friend and I with our lives."
by crankhead November 20, 2016

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A person who puts up definitions on urbandictionary
Stop being such a troll
by bob_bobbb July 18, 2016

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1a. Noun

One who posts a deliberately provocative message to a newsgroup or message board with the intention of causing maximum disruption and argument.

1b. Noun

A member of an internet forum who continually harangues and harasses others. Someone with nothing worthwhile to add to a certain conversation, but rather continually threadjacks, as well as thinks every member of the forum is talking about them and only them. Trolls often go by multiple names to circumvent getting banned.

2a. Noun

Sometimes compared to the Japanese ‘Oni’, a troll is a supernatural creature of Scandinavian folklore. Lives in hills, mountains, caves, or under bridges. They are stupid, large, brutish, hairy, long-nosed, and bug-eyed, and may also have multiple heads or horns. Trolls love to eat people, especially small children.

2b. Noun

An old, unattractive gay man who hits on young men, and cannot seem to understand that they want nothing to do with him.

3. Verb

To cruise for immediate sexual pleasure without commitment. Often characterized by an older man, seeking a younger man, or any man for immediate, often anonymous sexual contact.

4. Noun

5. Verb

To use acid and ectasy simulataneously. See trolling.

(Comes from ‘Tripping’ on acid, and ‘Rolling’ on ecstasy)

6. Noun

A toy popular in the 1980’s, a ‘troll’ is a cute little plastic trolls with fluffy, colored hair.

7. Noun

YOU KNOW WHAT IT IS YOU PIECE OF HOMESTUCK TRASH

note: most of the definitions are not mine
1a. I hate trolls!

1b. I hate trolls!

2a. It is said that a troll will turn to stone if caught by sunlight/daylight.

2b. That troll kept hitting on Rick.

3. He went trolling and picked up a 17-year-old boy recently at a gay club.

4. Hey, how's my computer able to speak in foreign languages?

5. Back in the 90's we used to troll at the raves.

6. I bought a 4-inch troll for my little sister on her birthday.

7. I know about the trolls even if i'm not at act 5 yet
by giannis makara/zahhak January 11, 2016

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A mythical creature inhabiting the vast planes of internet, creating and feeding on unintelligent people's hate, rage, misfortune and misinterpretation.

If you see a troll, proceed with caution, or you may get trolled. When you fail to recognize one and make conctact with it, you are already getting trolled.
Hide yo children, hide yo wife! they are easily trolled.
by Sunekus March 30, 2012

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A total jerk on the internet. Will tell things that are not real, and go off topic on forums, curse at other people, " fake and gay ", etc.
Troll: " fake and gay "
Un-infected person " You are such a troll. "
by Insane Rofler September 01, 2011

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A girl or woman who acts troll-like in public; hopping obnoxiously, eating like a pig, having poofy red tinted hair(usually not brushed), has an unbelievably HUGE appetite, mooches off of so called "friends" or colleges, nasty body usually fat and or with a celulite tummy, sloppy, easy, sometimes a slut.

TFL = Troll Free Life
Cory: "Man! That girl is a total troll... nasty!"
Jim: "Yeah I know, what a turn-off."

Jim: "Ew look at that fat troll hopping around in the hall."
Cory: "Gross. She should get that crazy hair thinned or somethin."

Cory: "DUDE I'M GLAD I HAVE A TFL."
Jim: "Mhm, halleluijah!"
by Trollz Slayer December 21, 2010

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A lowlife prick on the Internet who has no life and is spamming everyone online.
You fucking troll! Get off of my Facebook!
by Dick2Ass69 May 17, 2016

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