Character is a separate question from amount. Every dollar of income on a return has two attributes that matter: how much it is, and what kind it is. Character determines which rate schedule reaches it, whether a loss can offset it, and sometimes whether a deduction or credit is available against it. The reason "ordinary" needs a name at all is that a small number of provisions grant preferential treatment, and the tax system needs a word for everything those provisions do not reach. So ordinary is not a category with a list; it is the absence of a special rule.
Reading section 64 tells you something real about the drafting. If ordinary income were meant to be an affirmative category, section 64 would enumerate wages and interest. Instead it uses "includes" and addresses one narrow question: when property is sold, is the resulting gain capital or ordinary? Inventory, receivables and depreciable business property sold at a gain produce ordinary income rather than capital gain, and section 64 is the provision that says so. Everything else is ordinary because no provision made it anything else.
What carries ordinary character in practice. Compensation in every form, including bonuses, severance, and the spread recognized on exercising a non-qualified stock option. Business and self-employment profit. Interest, including from bonds, bank accounts and certificates of deposit. Ordinary dividends, meaning dividends that fail the qualified dividend tests. Rents and royalties. Short-term capital gains, which is what makes the twelve-month holding period consequential. Distributions from traditional retirement accounts, regardless of what the money was invested in inside the account, which is why a long-term gain earned inside a traditional individual retirement arrangement comes out as ordinary income. And depreciation recapture on the sale of business or rental property, which converts part of what feels like a capital gain back into ordinary income.
Ordinary income and earned income are different sets, and confusing them is expensive. Earned income means compensation for services: wages, salary, tips, and net earnings from self-employment. Ordinary income is a much larger set that also includes interest, rents and retirement distributions. The distinction is load-bearing in several places. Contributing to an individual retirement arrangement requires taxable compensation, not merely ordinary income, so a retiree living on interest and pension income has ordinary income and no contribution room. Self-employment tax applies to earned income from a business, not to ordinary investment income.
Being ordinary income and being taxed at ordinary rates are two separate facts, and the gap between them is stacking. A long-term capital gain is not ordinary income and is not taxed at the graduated rates. But the two interact, because the preferential layer is stacked on top of the ordinary layer when determining which capital gain rate applies. The consequence is that a change in ordinary income can change the rate on a capital gain that itself did not change, and the mechanics of that stacking belong to the taxable income page.