Taxable Income
  
See: Ordinary Income. See: Realized Gains. See: 401(k) Plan. See: IRA. See Spot. See Spot pay taxes.
Taxable income is just the "net" income you have to pay taxes on. So what's that "net" thing? Well, you might be a Lucy of Charlie Brown fame shrink. You run a psychiatry shop, but charge way more than a nickel an opinion. After a year, you've billed clients $300,000. But you had malpractice insurance, rent, computers, billing/merchant/credit card costs, an admin, and everything else...all totaling $120,000 in costs. So even though you had billings of $300k, you had taxable income of $180,000.
IRS, you blockhead.