What is a LOB application?

LOB (Line of Business) applications are enterprise level business application software which are critical to running complex processes.
A LOB application has generally simple UI but has big and complex OLTP processing going on.

According to Techtarget.com:

- An LOB (line-of-business) application is one of the set of critical computer applications that are vital to running an enterprise, such as accounting, supply chain management, and resource planning applications. LOB applications are usually large programs that contain a number of integrated capabilities and tie into databases and database management systems.

according to MSDN blog:

LOB applications are:

• Interactive--self explanatory
• Have multiple screens--the interaction happens on screens, and typically several of them are involved
• Domain specific--examples include finance, insurance, health care, telecom, and e-commerce applications
• OLTP--transaction processing rather than analytical processing
• Have relatively simple user interface/presentation--text fields, checkboxes, buttons
• Integrate with other systems that manage the data and execute the transactions--databases, systems of record, etc.