<span id="lifetime-tcx">`'tcx`</span>          |  The lifetime of the allocation arenas used by `TyCtxt`. Most data interned during a compilation session will use this lifetime with the exception of HIR data which uses the `'hir` lifetime. ([see more](../ty.md))
<span id="token">token</span>                  |  The smallest unit of parsing. Tokens are produced after lexing ([see more](../the-parser.md)).
<span id="tls">[TLS]</span>                    |  *Thread-local storage*. Variables may be defined so that each thread has its own copy (rather than all threads sharing the variable). This has some interactions with LLVM. Not all platforms support TLS.
<span id="trait-ref">trait reference, trait ref </span> |  The name of a trait along with a suitable list of generic arguments. ([see more](../traits/goals-and-clauses.md#trait-ref))
<span id="trans">trans</span>                  |  Short for _translation_, the code to translate MIR into LLVM IR. Renamed to [codegen](#codegen).
<span id="ty">`Ty`</span>                      |  The internal representation of a type. ([see more](../ty.md))
<span id="tyctxt">`TyCtxt`</span>              |  The data structure often referred to as [`tcx`](#tcx) in code which provides access to session data and the query system.
<span id="ufcs">UFCS</span>                    |  Short for _universal function call syntax_, this is an unambiguous syntax for calling a method. **Term no longer in use!** Prefer _fully-qualified path/syntax_. ([see more](../type-checking.md), [see the reference](https://doc.rust-lang.org/reference/expressions/call-expr.html#disambiguating-function-calls))
<span id="ut">uninhabited type</span>          |  A type which has _no_ values. This is not the same as a ZST, which has exactly 1 value. An example of an uninhabited type is `enum Foo {}`, which has no variants, and so, can never be created. The compiler can treat code that deals with uninhabited types as dead code, since there is no such value to be manipulated. `!` (the never type) is an uninhabited type. Uninhabited types are also called _empty types_.
<span id="upvar">upvar</span>                  |  A variable captured by a closure from outside the closure.
<span id="variance">variance</span>            |  Determines how changes to a generic parameter affect subtyping; for example, if `T` is a subtype of `U`, then `Vec<T>` is a subtype `Vec<U>` because `Vec` is _covariant_ in its generic parameter. See [the background chapter](./background.md#variance) for a more general explanation. See the [variance chapter](../variance.md) for an explanation of how type checking handles variance.
<span id="variant-idx">variant index</span>    |  In an enum, identifies a variant by assigning them indices starting at 0. This is purely internal and not to be confused with the ["discriminant"](#discriminant) which can be overwritten by the user (e.g. `enum Bool { True = 42, False = 0 }`).
<span id="wf">well-formedness</span>           |  Semantically: An expression that evaluates to meaningful result. In type systems: A type related construct which follows rules of the type system.
<span id="wide-ptr">wide pointer</span>        |  A pointer with additional metadata. See [fat pointer](#fat-ptr) for more.
<span id="zst">ZST</span>                      |  *Zero-sized type*. A type whose values have size 0 bytes. Since `2^0 = 1`, such types can have exactly one value. For example, `()` (unit) is a ZST. `struct Foo;` is also a ZST. The compiler can do some nice optimizations around ZSTs.