This note has all the common terminology related to different parts of source code.

Codebase is a collection of source code that is stored in one repository and deployed independently.

It doesn't mean a single codebase has to operate independently. Especially common in microservices architectural pattern.

How to introduce yourself to a new codebase:

1. How does the overall infrastructure look?
2. How is the code deployed?
3. Step debug through the whole program for a couple of executions.
4. Write a couple of black box end-to-end tests if there aren't any.

Other units of code:

• Expression: a collection of characters that are computed to produce a value. For example, 2+3 and b (variable) are expressions.
• Line: a collection of characters that ends with a new line.
• Statement: the smallest standalone element of a programming language that describes an action. One statement can be split on multiple lines. Many programming languages end statements with ;.
• Unit: the smallest grouping of statements that can be executed independently like a method or a function. This is where the term "unit test" comes from, testing the smallest possible executable part of a system.
• Module: the smallest grouping of units like a class in object-oriented programming.
• Component: division of a system defined by its architecture, not always defined by a programming language itself but for example Go has packages, Python has modules and Java has namespaces that can be used to define components.
• System: the entire codebase under study, not defined.

Sources

• Building Maintainable Sofware, O'Reilly