A computer application that only covers a single, distinct, area of functionality with little or no integration with other processes or other applications e.g. a simple calendar application or a business application that has complex functionality, but on for a distinct business process function such as creating and approving Purchase orders, but no direct integration/inter-dependencies with others functions, such as Financial Reporting or Stock Control in external applications or where activated as part of the same application (therefore not a Single Function Application).
The point is that Single Function Applications more likely to be SaaS products as they are less likely to need customisation or additional coding specifically for an individual installation (customer) or Integration testing.
Therefore Single Function Applications may be easily developed with traditional Agile values, rather than needing a Hybrid approach (e.g. a 'Waterfall' Integration and/or Regression Test phase after development Sprints and before Shipping each Release).

Anatidaephobia is defined as a pervasive, irrational fear that one is being watched by a duck. The anatidaephobic individual fears that no matter where they are or what they are doing, a duck watches.

Anatidaephobia is derived from the Greek word "anatidae", meaning ducks, geese or swans and "phobos" meaning fear.