a single word or group of words than can be used in just about any situation. a swiss army knife of the english language, a universal noun will have near limitless applications.
for an example of a universal noun, take the other urban dictionary listing word (and yes, i meant to refer to the listing for "word").
A versatile declaration, originating (more or less) in hip-hop culture.
"Word" has no single meaning, but is usually used to convey a casual sense of affirmation, acknowledgement, agreement, or to indicate that something has impressed you favorably.
Its usage among young blacks has been parodied ad nauseam among clueless suburban whites.
person 1: "Come on man, we're going to the store."
person 2: "Word." (acknowledging and agreeing with)
person 1: "Word!" (conveying "I've got important news to tell you")
person 2: "What's up man?!"
person 1: "Yo, I wrecked my car last night."
person 2: "Word?" ("You speak the truth?")
person 1: "Word." ("I speak the truth.")
person 3: "Word..." (acknowledging the event, as well as the other's financial and emotional loss)
a statement of indifference
1) After any sentence
2) After any statement of any kind (i.e. angry statement, statement of love...)
3) Just about anywhere you feel like putting it.
4) Don't abuse the power of word. Use it properly. And randomly.
Person 1: What did you do last night?
Person 2: Went to the movies. Word.
Person 1: Word.
Person 1: What did you do last night? Word.
Person 2: I went to the AMC theatre and watched a movie.
Person 1: Went to the word movies huh? Word, word. Word.