The commonly accepted term for the various available classes that a person can potentially obtain within the Infinite Labyrinth. Each Profession determines what skills that can be earned or improved, while allowing people to improve themselves.

Tiers represent broad categories within the Labyrinth. Each Zone has an associated Tier with it, which determines how hard it is, which Professions can be obtained at its centre, and how to access it.

The higher the tier, the more powerful the Profession is.

Skills are individual abilities, both passive, active, or triggered, that Professionals can acquire and use more or less instinctively. Each skill belongs to a specific sphere and type (like Offense/Aether), and can only be acquired if the person's current Profession matches the skill.

Zones are individual pocket dimensions within the Infinite Labyrinth. Each one is self-contained, with huge variability. Zones have a Plaza at their centre, and Gates at their periphery.

Gates are the means by which Professionals can travel between individual zones within the Infinite Labyrinth.

There are two types of Gates: Great Gilded Gates, which connect Earth with a tier-one Zone, and normal Gates that connect two individual Zones. Great Gilded Gates can be crossed by anyone qualified to be a Professional; if the person does not yet have a Profession, they will be transported to the Zone's Plaza instead and allocated one. Normal Gates can be crossed only if they have been crossed before, or if the person has completed as many lairs in the original zone as there are tiers at their destination (eg, one lair required for a tier-one zone, two for a tier-two, etc).

A general ability of Professionals, available only while in the Labyrinth. It returns you to the latest Recall position, which can be set by touching a Recall Stone - a unique sculpture, typically located next to the Plaza. Cannot be used if you are not fully regenerating, and requires one entire day cycle to recover.

Lair is a category of areas within a Zone of the Labyrinth. Lairs can be outdoor or indoor and are characterized by a higher density of enemies, who appear in packs and of higher difficulty.

Lairs include treasure chests, guarded by an enemy or pack significantly harder than the rest. The chest holds one or more items, and represent the only reliable source of equipment within the Labyrinth.

Lairs have to be completed to enable Gate travel. Crossing a Gate to a tier N zone requires completing N lairs within the previous zone. Completion requires defeating the last guardian of the lair, and a significant amount of enemies besides. Having more than six Professionals involved in attempting a lair dilutes this, resulting in some of the Professionals not completing the lair, despite having fought its guardians.

The Plaza is the central point of each Zone. It appears as a large marble circular structure, surrounded by doric-style pillars with a circular lintel joining them. The Plaza has no ceiling and features an arch entrance which is widely considered to be symbolic, as the structure is entirely open on all sides.

Plazas are where Professionals obtain their original Profession and all subsequent ones. Plazas offer only Professions of the tier of their zone and a limited subset of those beyond tier-one.

Milestones are specific levels within the progression of a Profession.

Each Milestone grants additional points of Potential used to unlock future Professions, and allow you to increase the rank of one skill with at least one common sphere with the Profession. Each skill can receive only one Milestone rank per Profession, and unspent ranks cannot be allocated unless your current Profession is the same as the Milestone.

Potential indicates the measure of a Professional's capacity in a specific area. There are ten Potentials, broadly five physical and five non-physical.

Baseline people have a score comprised between 10 and 20 in each Potential, with an average of 15 and a bell-shaped distribution curve. Tier-one Professions require a score of 18 in a Potential, meaning less than one person in 375 qualifies as a Professional.

Labyrinth equipment can increase an effective Potential score, but that increases only translate into ability, and not qualifications for Professions, or the rare no-level items could be used to allow an unqualified person to step into the Labyrinth.

Adjustment is an anomalous Milestone, granted to unqualified people who were thrown into the Labyrinth but could not be safely turned away.

Unlike normal Milestones, this one is not associated with a visible Profession and grows only in specific circumstances, including taking a new Profession. Each Milestone adds a stacking bonus to every Potential, accelerates levelling, and apparently grants special abilities rather than allocatable skill ranks.

One of four measures: Health, Mind, Endurance and Aether. The first two represent levels of physical integrity and mental stability, while the latter are pools of physical and magical energies used for activities.

Dropping to zero health occurs when one dies. In that event, another Professional can Sacrifice as much health as required to bring back the deceased to life. This must be done with minutes, one per level of the deceased.

Dropping to zero mind makes one go crazy. Crazed Professionals behave irrationally and usually attack anything they can, including fellow teammates. A crazed Professional is not immune to offensive abilities from those same teammates. Craziness ends when enough mind is recovered, or the person dies.

Any skill requiring endurance or aether cannot be used or triggered if there is not enough in the appropriate pool. Additionally, endurance can be consumed by non-skill activities like running or using weapons and having less aether than your defence rating reduces resistance to magical damage (aether is not consumed when suffering magical damage).