A trust or corporate trust is a large grouping of business interests with significant market power, which may be embodied as a corporation or as a group of corporations that cooperate with one another in various ways. These ways can include constituting a trade association, owning participating interests in one another, constituting a corporate group (sometimes specifically a conglomerate), or combinations thereof. The term trust is often used in a historical sense to refer to monopolies or near-monopolies in the United States during the Second Industrial Revolution in the 19th century and early 20th century. The use of corporate trusts during this period is the historical reason for the name "antitrust law".

In the broader sense of the term, relating to trust law, a trust is a legal arrangement based on principles developed and recognised over centuries in English law, specifically in equity, by which one party conveys legal possession and title of certain property to a second party, called a trustee. The trustee holds the property, while any benefit from the property accrues to another person, the beneficiary. Trusts are commonly used to hold inheritances for the benefit of children and other family members, for example. In business, such trusts, with corporate entities as the trustees, have sometimes been used to combine several large businesses in order to exert complete control over a market, which is how the narrower sense of the term grew out of the broader sense.

In the United States, the use of corporate trusts died out in the early 20th century as U.S. states passed laws making it easier to create new corporations.

The OED (Oxford English Dictionary) dates use of the word trust in a business organization sense from 1825.

The business or "corporate" trust came into use in the 19th-century United States, during the Gilded Age, as a legal device to consolidate industrial activity across state lines. In 1882 John D. Rockefeller and other owners of Standard Oil faced several obstacles to managing and profiting from their large oil refining business. The existing approach of separately owning and dealing with several companies in each state was unwieldy, often resulting in turf battles and non-uniform practices. Furthermore, the Pennsylvania legislature proposed to tax out-of-state corporations on their entire business activity. Concerned that other states could follow, Standard Oil had its attorney Samuel C. T. Dodd adapt the common law instrument of a trust to avoid cross-state taxation and to impose a single management hierarchy. The Standard Oil Trust was formed pursuant to a trust agreement in which the individual shareholders of many separate corporations agreed to convey their shares to the trust; it ended up entirely owning 14 corporations and also exercised majority control over 26 others. Nine individuals held trust certificates and acted as the trust's board of trustees. One of those trustees, Rockefeller himself, held 41% of the trust certificates; the next most powerful trustee held about 13%. This trust became a model for other industries.

An 1888 article explained the difference between trusts in the traditional sense and the new corporate trusts:

A trust is ... simply the case of one person holding the title of property, whether land or chattels, for the benefit of another, termed a beneficiary. Nothing can be more common or more useful. But the word is now loosely applied to a certain class of commercial agreements and, by reason of a popular and unreasoning dread of their effect, the term itself has become contaminated. This is unfortunate, for it is difficult to find a substitute for it. There may, of course, be illegal trusts; but a trust in and by itself is not illegal: when resorted to for a proper purpose, it has been for centuries enforced by courts of justice, and is, in fact, the creature of a court of equity.

— Theodore Dwight, Political Science Quarterly