# Vocabulary for loops

Canonical: https://loopmaster-ai.pages.dev/learn/loop-or-not

Loops are hard to talk about because the words around them are overloaded. Four terms carry most of the weight on this site. Learn them once and every later lesson gets shorter.

## Action, turn, pass, run

**Action** — the smallest unit: one tool call, one command, one request. "Ran the test suite" is an action. Actions succeed or fail; they don't decide anything.

**Turn** — one stretch of model work: the model reads what's in front of it, takes some actions, and hands back a result. A turn can contain many actions. In a chat window, one reply is one turn.

**Pass** — one full trip around the loop: do the work, run the check, record a verdict. A pass usually contains one or more turns. This is the level where judgment lives — a turn produces output, a pass produces a verdict.

**Run** — everything from start to stop: all the passes from the first one to the one where a stop rule fired. A run ends exactly once, for a stated reason.

The containment reads bottom-up: actions make up turns, turns make up passes, passes make up a run.

A worked example. A loop is fixing a failing test. On pass three, the model reads the latest error, edits a file, and reruns the suite — that's one turn containing three actions. The suite still fails, so the check records "not yet" and the harness starts pass four. On pass five the suite goes green, the check records it, and the stop rule fires. The run is over: five passes, ended because the check passed.

## The nesting law

Loops stack: one loop's pass can kick off another loop's entire run. A weekly "keep dependencies current" loop might, in a single pass, decide that one library needs upgrading — and launch a whole run of an upgrade loop to do it.

The rule that keeps the vocabulary straight:

**A pass at one layer typically contains a run at the layer below.**

So "pass" and "run" aren't sizes — they're positions. The same afternoon of work is a run to the loop that did it, and part of one pass to the loop that asked for it.

One caution: the law says *typically contains*, not *must contain*. A steering pass — one pass of a loop that revisits goals, which a later lesson defines properly — may launch several goal-loop runs and review them asynchronously as they finish. Don't force symmetry onto the layers; the law describes the common shape, not a requirement that every pass wait on exactly one tidy run below it.

## Vendor words for the same things

Every vendor names these pieces differently — steps, iterations, turns, sessions, episodes, traces, trajectories. The concepts are stable even when the labels aren't. When you hit an unfamiliar term in someone's docs, the [glossary](/learn/glossary) maps vendor vocabulary onto the words used here.

## Do this next

Vocabulary sticks when you use it on a real run. Build a small loop and narrate it as it goes — name each action, each turn, each pass, and the moment the run ends: [start from the Quick Start](/quick-starts/qs1-first-task-loop).