Two features of the definition explain almost everything that follows from it. The first is that it is functional rather than formal: Congress listed instruments but did not stop at instruments, adding "investment contract" and "any interest or instrument commonly known as a 'security'" so that the statute would reach arrangements nobody had thought of yet. The second is the opening qualifier, "unless the context otherwise requires", which means the definition is not mechanical even on its own terms.
The Supreme Court supplied the operative test for the catch-all in SEC v. W. J. Howey Co. in 1946. The case concerned tracts of a Florida citrus grove sold to out-of-state buyers together with a service contract under which the seller cultivated and marketed the crop and paid each buyer a share of the proceeds. The Court held that this was an investment contract, and therefore a security, even though what changed hands on paper was land. Its reasoning is the durable part: the term had been "broadly construed by state courts so as to afford the investing public a full measure of protection", and "form was disregarded for substance and emphasis was placed upon economic reality."
The Court's own formulation is worth reading twice: "an investment contract for purposes of the Securities Act means a contract, transaction or scheme whereby a person invests his money in a common enterprise and is led to expect profits solely from the efforts of the promoter or a third party, it being immaterial whether the shares in the enterprise are evidenced by formal certificates or by nominal interests in the physical assets employed in the enterprise." It restated the test more compactly at the end: "the test is whether the scheme involves an investment of money in a common enterprise with profits to come solely from the efforts of others." Whether a particular arrangement satisfies it is decided on the facts of that arrangement, not on its label.
The consequences of the classification are the reason it is fought over. If an arrangement is a security, then offering or selling it requires a registration statement unless an exemption applies, and registration means audited financial statements and a prospectus. The antifraud provisions attach, which is what gives the SEC and private plaintiffs a cause of action for misstatements. The people selling it generally have to be registered as brokers or work for a firm that is. State blue-sky laws add a further layer. If it is not a security, none of that follows from the federal securities laws, though other law may of course apply.
The Securities Act's definition is not the only one. The Securities Exchange Act of 1934 has its own, at 15 U.S.C. 78c(a)(10), which tracks the 1933 Act closely but is not identical: it omits "evidence of indebtedness", it does not include the oil, gas and mineral-rights item in the same terms, and it ends with an express exclusion, providing that the term "shall not include currency or any note, draft, bill of exchange, or banker's acceptance which has a maturity at the time of issuance of not exceeding nine months, exclusive of days of grace, or any renewal thereof the maturity of which is likewise limited." That nine-month carve-out is why short-term commercial paper is generally discussed outside the securities framework. When a source says something "is a security", the useful follow-up question is: under which statute, and for what purpose.