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# Defining Tests in Distributional

What's in a test?

Tests are the core mechanism for asserting acceptable app behavior within Distributional.  In this section, we introduce the necessary tools and explain how they can be used -- either with guidance from Distributional or by users based on unique needs.

At its core, a **Test** is a combination of a **Statistic**, derived from a run, which defines some behavior of the run, and an **Assertion** which enumerates the acceptable values of that statistic (and, thus, the acceptable behavior of a run). The run under consideration in a test is referred to as the **Experiment** run; often, there will also be a **Baseline** run used for comparison.

**Test Tags** can be created and applied to tests to group them together for, among other purposes, signaling the shared purpose of several tests (e.g., tests for text sentiment).

When a group of tests is executed on a chosen Experiment and Baseline Run, the output is a **Test Session** containing which assertions Pass and Fail.