Why do we allow ourselves to buy into the landslide of marketing? The chosen words of game designers have become used as they are not employed in a manner consistent with their meaning. Designers are removing value from the words, not adding to them! Desensitization, anyone?
Diving Right In
Common gear means gear within 1? 2? 3? standard deviations of the mean.
Superior means above the center-mass.
Rare means (perhaps more exclusively, above the limits of standard deviation provided by the common definition) not common at all.
Epic means having been acquired through a momentous and noteworthy act.
Legendary means existing only in hearsay.. something people speak of but no one except some elder from an isolationist tribe in the far hills has any visual confirmation of that also hasn't been seen in a century (see also: unique).
Without Further Ado
A player is one.
A party is a collection of players able to, at a glance, identify precisely the number of players in the collective.
A group is a collection of players able to, at a glance, approximate within 10% the number of players in the collective.
A mini-raid is a collection of players unable to, at a glance, approximate within 10% the number of players in the collective, but able to given a quick (5 seconds?) headcount.
A raid is a collection of players unable to approximate within 10% the number of players in the collective, and also unable to given a quick headcount.
A horde is straight un-tally-able.
And Then Some
A title invokes a sense of respect. (?)
A quest invokes a sense of adventure, once-in-a-lifetime life-altering journey. (?)
A bag invokes a volume limit, able to carry objects of smaller size(s). (?)
An achievement invokes an earning process and is this uncommon. (?)
Experience, talent (trees), home/inn, ... all hold real value and get tossed around.
Chicken Or Egg (Edit)
Consider the growth of a child. Upon entering this world through the magic of birth, no concept of 'words' is known. Sounds, sound imitation win the day. A child knows what something is before they have a label for it. This is done throughout life. Some individuals/groups know this, and base their method of learning specifically around this fact.
When people use labels, they are trying to incur that definition in the mind of their audience. It is the definition that yields the word. Words are used to call definitions. This is the organization of the two-way street.
When something needs a handle to help others relate to it, one gives a name to a meaning. But if names and/or meanings already exist...
Using a name that incurs a definition does not mean the thing that has been named inherits the definition! By calling a carrot blue, it does not become a bluish hue in color. By calling sandpaper smooth, it does not become a uniformly level surface at microscopic level. It is the thing's definition that gives it a name, not the other way around. Care to abusing the order of horse:cart!
What to watch for: collaborative series on how far removed defining features have become from their handles.