A citizen developer is an employee without formal programming training - often a salesperson, an operations person or a small business owner - who uses no-code or low-code tools to build their own apps, databases and automations for their team. They don't need an IT department or an outside contractor to do it.
The term arose as a counterpart to "shadow IT" (unapproved, risky use of tools outside company policy). Today, citizen development is increasingly a deliberately supported practice - a company gives the team a tool with clear guardrails (permissions, backups and change history are handled centrally by the platform) and lets people build solutions for their own needs.
A typical example: the owner of a service business who spends one afternoon building a job tracker with automatic invoicing, without hiring a developer.
Related: I built our business system in an afternoon. No developer., Custom CRM without a developer: a step-by-step guide, Access rights and data visibility.