Ketone is isomeric with an organic compound containing at least one aldehyde group
Isomers have same molecular formula but different functional groups due to variation in positioning of certain atoms within molecules.
Let us consider 1-propanal (contains one aldehyde group). Acetone is isomeric with 1-propanal as both have molecular formula . But 1-propanal contains aldehyde (-CHO) functional group and acetone contains keto (-CO-) functional group. Isomerism appears between these two compound due to variable positioning of oxygen atom.
Therefore they are isomeric to each other.
An image has been attached below for more clarification.