Whatsit and X

Whatsit noun - A small article the actual name of which one either does not know or cannot remember.

X and whatsit are semantically related. In some cases you can use "X" instead a noun "Whatsit".

X noun - The cardinal number that is the sum of nine and one; the base of the decimal system.

Whatsit and x are semantically related. Sometimes you can use "Whatsit" instead a noun "X".