Scratchpad is one of those words
we use very often in our communication
but is not considered part of regular vocabulary
In day-to-day life, its a sticky note at times. A regular but rough usage small sized paper pad often.
But to me, one of the main requirements of any practical scratchpad is the ability to be wiped clean and reused. To this effect, programs like handwrite for palm fit my personal definiiton like a glove.
In computer jargon - scratchpad is used (not very commonly though) for a high-speed internal memory used for temporary storage of preliminary information. (According to WordNet ® 1.6, © 1997 Princeton University)