A portable application is a software program that does not require any kind of formal "installation" onto a computer's permanent storage device to be executed, and can be stored on a removable storage device such as a USB flash drive and used on multiple computers.  USB flash drives are typically small, lightweight, removable and rewritable. 

This does not mean that it can be taken and used on a different operating system, processing platform, or another computer with completely different hardware (i.e., those that are not compatible with the software as stated by its requirements), so it is not to be confused with the concept of software portability, which is the ability for software to be run or compiled with little modification on diverse computing platforms. Ideally it can be configured to read its configuration from the same location as the software.