What is disposable income?
Disposable income is, loosely speaking, the portion of your income that you can do whatever you want with. It's not limited to fun money, but it certainly includes it. When most people talk about disposable income, they mean whatever is left after taxes and mandatory contributions that you cannot opt out of, such as to Social Security and Medicare, are taken from your paycheck.
Other pre-tax contributions, like 401k deductions or insurance premiums, are not considered mandatory payments, so they are included as part of your disposable income. After all, you could elect to stop contributing to your retirement account or cancel your insurance.