Input validation refers to the process of validating all the input to an application before using it. Input validation is absolutely critical to application security, and most application risks involve tainted input at some level.
Many applications do not plan input validation, and leave it up to the individual developers. This is a recipe for disaster, as different developers will certainly all choose a different approach, and many will simply leave it out in the pursuit of more interesting development.
See Data_Validation for more.