Definition:

A system's usability is improved and the learnability is increased when it functions in a consistent way. Users of your system can focus on completing their task if they can establish how it works by experiencing it before, instead of rethinking their actions at every step.

Interaction Design:

Consistency is one of the main pillars of interaction design. The methods to restart a process, undo or redo an action, navigate between steps or enter information should be consistent. This principle can get tricky very quickly — a form should look like a form, but not all forms should look the same. Forms that are saved automatically vs queued before save should look different. A powerful way to get a user's attention is to break this rule. Modal dialog boxes that are displayed before a form is submitted can protect the user from doing something potentially dangerous.

User Experience Design:

Consistency in experience is important to every successful brand. Southwest Airlines is a company that has recently ramped (get it?) up it's customer experience by having two consistent, unchanging goals — to be the low-cost airline and to not take advantage of passengers with extraneous fees. Southwest has started to run commercials in the US that claim no bag fees. No weight limits. No size limits. No bag fees. Period. They've made the decision to meet their operational budget by streamlining other processes instead of charging the customer.