Anachronism
A temporal discord between coexisting elements; that is, when different parts of a scene appear to not be from the same time. Borges' irreality lends itself to this, as well as the act of Pierre Menard recreating Don Quixote.
Chaos
At points in the text, Borge's use of chaos references a concept closer to chaos theory than our colloquial use of the term - see, for instance, The Lottery of Babylon (104). In this sense, chaos refers to systems which are extremely sensitive and variable based on their input. It adds another dimension to the cosmic origin of chance as an underlying component of the universe.
Dialectic
A philosophical discipline that approaches education and the pursuit of truth through the teacher asking questions of the students. The protagonist of The Circular Ruins utilizes this approach to form his son.
Forms
Forms, with a capital "F," refer to Plato's theory of reality. According to Plato, the most "real" things are ideal Forms of objects and Ideas, which exist in an ideal world and are only manifested in our world as imperfect copies. Forms figure into Borges' ideas about identity and the notion of ownership.
Heresiarch
The founder of a heretical doctrine and/or the leader of a sect which follows that doctrine. In Borges' imagined societies, such as Uqbar and the Library of Babel, heresiarchs play a significant role in fleshing out the societal views on what is acceptable, what is not, and how humanity has evolved over time.
Nihilism
A philosophical doctrine which denies that there is purpose in the universe, popularized by Friedrich Nietzsche. Because the notion of God directly relates to purpose, nihilism typically requires either a rejection of God's existence altogether, or some explanation of God's absence - e.g., that God is dead.
Omnipotence
A term referring to the quality of something with literally boundless potential - e.g., an omnipotent God. Because Borges deals in matters of infinity, omnipotent forces frequently arise. For instance, in the brief moment when Yu Tsun perceives time as an infinite labyrinth, one could argue that he is omnipotent (122).
Pullulation
Germination or sprouting. Borges uses this term to describe the natural inception of ideas and notions, fitting into the Platonic conception of ideas as independent entities.
Sophistry
The use of rhetoric to defend an argument with reasoning that is seemingly plausible, but which is not actually sound.
Totality
In logic, a total set is one in which all formulas are derivable. Putting aside the math itself, the logical concept is important to Borges primarily in The Library of Babel, because the total nature of the Library means that all possible books exist. The notion is an important derivative of one of Borges' favorite themes - infinity - and essentially allows for all possible knowledge to exist in abstract space, an inspiring notion because there is virtually no limit to what we can know.