diff --git a/doc/glossary.md b/doc/glossary.md
index af399e1..0218824 100644
--- a/doc/glossary.md
+++ b/doc/glossary.md
@@ -57,6 +57,12 @@
 A unit of time. One [flick](https://github.com/OculusVR/Flicks) (frame-tick) is
 `1 / 705_600_000` of a second.
 
+#### Hermetic Function
+
+A function that can modify only local state (to store the result of
+computation), not global or system state. See the
+[hermeticity](/doc/note/hermeticity.md) note for more details.
+
 #### Interval Arithmetic
 
 Arithmetic on numerical ranges. See the [interval
diff --git a/doc/note/auxiliary-code.md b/doc/note/auxiliary-code.md
index c3f9dcf..1f7086b 100644
--- a/doc/note/auxiliary-code.md
+++ b/doc/note/auxiliary-code.md
@@ -1,11 +1,12 @@
 # Auxiliary Code
 
 Wuffs is a [memory-safe](/doc/note/memory-safety.md) programming language,
-achieving that safety in part because Wuffs code doesn't even have the
-*capability* to dynamically allocate and free memory. Wuffs code is also
-transpiled to C (which is very portable and easy to bind to other, higher-level
-languages), but C lacks modern conveniences like a built-in string type that's
-safe and easy to use.
+achieving that safety in part because Wuffs code is
+[hermetic](/doc/note/hermeticity.md) and doesn't even have the *capability* to
+dynamically allocate and free memory. Wuffs code is also transpiled to C (which
+is very portable and easy to bind to other, higher-level languages), but C
+lacks modern conveniences like a built-in string type that's safe and easy to
+use.
 
 Wuffs' C/C++ form (a "single file library") also contains auxiliary C++ code
 (in the `wuffs_aux` namespace) that compensates for that. For example, the JSON
diff --git a/doc/note/hermeticity.md b/doc/note/hermeticity.md
new file mode 100644
index 0000000..29168d6
--- /dev/null
+++ b/doc/note/hermeticity.md
@@ -0,0 +1,91 @@
+# Hermeticity
+
+Hermetic functions (or hermetic methods) are pieces of code that can modify
+only local state (including its arguments) to store the result of computation.
+They cannot modify global state (including thread-related state like mutexes)
+or make actual or virtual system calls (including allocating memory or querying
+timers). They can call other hermetic functions but any recursion must be
+declaratively finite (so that a worst case stack size requirement can be
+computed at compile time) and they cannot invoke user-supplied callbacks.
+
+
+## Purity
+
+Hermeticity is similar to
+[purity](https://en.wikipedia.org/wiki/Pure_function), although the exact
+definition of purity [depends on who you
+ask](https://twitter.com/radokirov/status/1097325661658570752). While pure
+functions are usually defined as without side effects, hermetic functions can
+modify its arguments' state (not just modify local variables).
+
+For example, in Go notation, `func pureSort(original []int) (sorted []int)`
+could take a slice of `int`s and return *a new slice* containing the elements
+in sorted order - the `original` slice is not modified. In comparison, `func
+hermeticSort(dst []int, src []int) (dstWasLongEnough bool)` could copy `src`'s
+elements (in sorted order) to modify *a caller-supplied slice* `dst`, provided
+that `dst` was long enough.
+
+This distinction is somewhat fuzzy if you think of the pass-a-`dst`-slice
+mechanism as merely a performance optimization for 'returning' large values via
+an out parameter. Still, hermetic *methods* are allowed to modify the receiver
+argument's state, which is separate from return values or out parameters.
+
+For example, `func (otp *oneTimePad) Transform(dst []byte, src []byte, atEOF
+bool) (nDst int, nSrc int, err error)` could combine a [one-time
+pad](https://en.wikipedia.org/wiki/One-time_pad) with an input stream, writing
+the result to an output stream. Calling `Transform` performs some compute but
+it also updates the receiver `otp`'s state to track how much of the one-time
+pad remains after successive calls during streaming I/O. This
+[`Transform`](https://pkg.go.dev/golang.org/x/text/transform?tab=doc#Transformer)
+method can be hermetic without being pure.
+
+
+## In-Process Sandboxing
+
+Hermeticity is similar to process-level sandboxing (like Linux's
+`SECCOMP_MODE_STRICT` sandbox) but for hermetic functions, both caller and
+callee are in the same process. The motivation is the same - to compute on
+untrusted data without security-compromising surprises, even if there are bugs
+in the computation code - but the mechanism is different.
+
+For `seccomp`, the operating system enforces the restriction (at run-time) and
+communication (literally IPC or Inter-Process Communication) is heavyweight,
+asynchronous and complex.
+
+For Wuffs, the compiler enforces [memory-safety](/doc/note/memory-safety.md)
+(at compile time) and communication (a function call) is lightweight,
+synchronous and simple. [Wuffs the Language](/doc/wuffs-the-language.md) is
+deliberately unpowerful: there are no global variables, no `unsafe` keyword, no
+FFI (Foreign Function Interface) or system call facility, no user-supplied
+callbacks, no allocation or de-allocation of memory and no panicking or
+throwing exceptions (including for out-of-memory). Wuffs is designed for
+writing hermetic, secure libraries, not complete programs, and **with less
+power comes easier proof of safety**.
+
+In comparison to compiling C/C++ code with WebAssembly (which can be restricted
+to only compute), both Wuffs and Wasm allow for hermetic libraries within
+larger (memory-unsafe) C/C++ programs. The difference is that Wasm lets you
+re-use existing C/C++ libraries (albeit with a run-time [performance
+penalty](https://kripken.github.io/blog/wasm/2020/07/27/wasmboxc.html)) but
+Wuffs performs [on par with C
+code](https://github.com/google/wuffs/blob/master/doc/benchmarks.md) (albeit
+requiring re-writing those libraries).
+
+
+## Termination
+
+Wuffs functions are not guaranteed to terminate. We do not solve the [Halting
+Problem](https://en.wikipedia.org/wiki/Halting_problem). Furthermore, while
+there is a difference in theory between a function that doesn't terminate and
+one that terminates in 99 years, in practice there is not. Systems that already
+need to cope with the latter can also cope with the former.
+
+
+## Timing and Other Attacks
+
+Technically, 'hermetic' code can still affect other threads or processes
+indirectly: modifying what's in hardware caches, OS page tables or that
+spending CPU quota leads to a scheduler switch. Wuffs does not guarantee that
+code is safe from timing or speculative execution attacks. Nonetheless, Wuffs
+code does not have access to any clocks. Any Wuffs computation on "the current
+time" requires the caller to explicitly pass in that information.