<span id="subst">subst</span>                  |  The act of _substituting_ the generic parameters inside of a type, constant expression, etc. with concrete generic arguments by supplying [substs](#substs). Nowadays referred to as _instantiating_ in the compiler.
<span id="substs">substs</span>                |  The _substitutions_ for a given generic item (e.g. the `i32`, `u32` in `HashMap<i32, u32>`). Nowadays referred to as the list of _generic arguments_ in the compiler (but note that strictly speaking these two concepts differ, see the literature).
<span id="sysroot">sysroot</span>              |  The directory for build artifacts that are loaded by the compiler at runtime. ([see more](../building/bootstrapping/what-bootstrapping-does.html#what-is-a-sysroot))
<span id="tag">tag</span>                      |  The "tag" of an enum/generator encodes the [discriminant](#discriminant) of the active variant/state.  Tags can either be "direct" (simply storing the discriminant in a field) or use a ["niche"](#niche).
<span id="tait">TAIT</span>                    |  A type-alias `impl Trait`. Introduced in [RFC 2515](https://rust-lang.github.io/rfcs/2515-type_alias_impl_trait.html).
<span id="tcx">`tcx`</span>                    |  Standard variable name for the "typing context" (`TyCtxt`), main data structure of the compiler. ([see more](../ty.md))
<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))