NN Group defines a complex application as

"any application supporting broad, unstructured goals or nonlinear workflows.

In addition, complex apps frequently:

• Support highly trained users with specialized knowledge
• Help users navigate and manage large underlying data sets and enable advanced sensemaking or data analysis
• Support problem solving or end goals with unknown or variable underlying tasks
• Require handoff or collaboration among multiple roles, tools, or platforms
• Mitigate the risks of executing high-impact (or high-value) tasks, where high loss (e.g., revenue or even lives) is at stake"

Many complex applications are focused on supporting specialized domains, where the work requires specialized background and training.

Contrast this against generalized applications, where NN Group defines them as having the ability to "enable nonspecialized users to complete largely discrete, linear tasks organized around well-structured goals."