Four sources, four different statutes, and the answers do not resemble each other.
An inheritance or a gift is excluded from gross income by section 102(a): "gross income does not include the value of property acquired by gift, bequest, devise, or inheritance." Two limits follow it. Section 102(b) does not exclude the income the property later produces, so an inherited portfolio arrives untaxed and its dividends are taxable. And section 102(c) removes employer-to-employee transfers from the exclusion entirely, so a payment from an employer is compensation whatever it is called.
A life insurance death benefit is excluded by section 101(a)(1), which keeps out of gross income "amounts received (whether in a single sum or otherwise) under a life insurance contract, if such amounts are paid by reason of the death of the insured." That exclusion is expressly subject to the transfer-for-value rules in the following paragraphs, which can reduce it where a policy was bought from someone else.
A personal injury recovery is excluded by section 104(a)(2), and this is where the common shorthand fails hardest. See below.
A prize, award or lottery win runs the other way. Section 74(a) states that "gross income includes amounts received as prizes and awards." The exceptions are narrow: section 74(b) covers certain achievement prizes the recipient was selected for without entering and which the payor transfers straight to a charity on the recipient's designation, so taking the money and donating it afterwards does not qualify; and section 74(c) covers certain employee achievement awards. Neither describes a lottery ticket.
Why "a settlement is tax-free" is wrong three times over, and all three are in section 104(a) itself. First, the exclusion in paragraph (2) reaches "the amount of any damages (other than punitive damages)": punitive damages are taxable. Second, the injury must be physical. The statute says "personal physical injuries or physical sickness," and its own flush language adds that "emotional distress shall not be treated as a physical injury or physical sickness," except to the extent of damages not exceeding amounts paid for medical care attributable to that distress. Third, the whole of subsection (a) opens with a clawback: the exclusions apply "except in the case of amounts attributable to (and not in excess of) deductions allowed under section 213 for any prior taxable year," so medical costs already deducted in an earlier year make that much of the recovery taxable. A single settlement payment can therefore contain excluded and taxable components, and the allocation in the settlement document is what determines which is which.
The sources this page deliberately does not answer. Severance from an employer, damages for lost wages, a distribution from an inherited retirement account, cancelled debt, cryptocurrency proceeds and the sale of a structured settlement each have their own rule, and several of them are commonly misdescribed. Where a large sum does not fall cleanly into one of the four categories above, the treatment has to be established for that specific source rather than inferred from the fact that the money was unexpected.
What to do first, which is mostly nothing. Almost every decision a large sum invites is reversible, and the two or three that are not tend to be made first: paying off a mortgage, buying property, lending or giving money to family, or committing to a product with a surrender charge. Establishing the tax character, setting aside the tax that will be owed, and parking the balance somewhere safe costs nothing and forecloses nothing. The behavioral risk is separately documented: money labeled as different from ordinary income tends to be spent differently, and a permanently higher spending level funded by a one-time sum is the most common way a windfall disappears without anyone deciding to spend it.
One naming note, because the two uses collide. Social Security's Windfall Elimination Provision was an unrelated rule reducing benefits for people with a pension from work not covered by Social Security. It was repealed, and it has nothing to do with receiving a lump sum.