A hive is a city that is an infection on the face of a planet. It is somewhere where too much is. It has populations that cannot countenance the sun, never minding that they have never seen it. it has sumps that make oceans seem puddles, spires that taunt heaven, it creaks with stresses of fathoms and leagues and measurements no man should write down. It is a seething mass of humanity, a wound wound round with huddled masses, the tired and desperate stretching their hands to support boots stamping in their face forever.
A city becomes a hive when it ceases to be administered, ceases to be controlled, but becomes a fed thing. When it no longer has a vulgar argot or a slang but a canonicity of tongue, a stack of languages predicated upon the one foundation. A hive is a city that can withstand invasion with the same grace that a restaurant bears a boorish assemblage for an annunciation, when its levies are so numerous that they bridge rivers with the bodies of their fallen, when it takes whole worlds to sate their demand for boots.
More pointedly, indeed, literally so, a hive city is one where so many centres of population have been assembled in stacked and interlocked arcologies, cell upon cell, block upon block, that they stack and inconvenience themselves to the stars.
Five towns may join to form a city. Three cities may join to form a conurbation. Half a thousand cities stacked up like gyre-worms devouring a wounded sibling, that is a hive.
If a ventilation failure does not kill a short legion of possible recruits to the guard; if there are not nobles who have grown fat from millenia of profit from concessions in the printing of transport tickets; if the Astropaths need not be shielded from the keening madness of that many trapped souls to communicate with the universe outside; if more than one twenty-third of the population can express clearly the notion of a definable 'not-here' with facts more concrete than 'it is not-here, it is the other-where'; if one can look upon it and not weep; then it is not a hive city. Else, well, make your mark, friend, you are hive-bound.