"Disposable" is an adjective meaning something not reuseable but is disposed of after use. Many people now use the term as a noun or substantive, i.e. "a disposable" but in reality this is still an adjective as the noun (product, nappy, etc.) is implied. Disposable income is the amount of money left over from one's salary or pay for spending, saving or whatever, after all living costs have been taken out.