In a more formal legal sense it means something that is unworthy of the law's attention.
In risk assessment it refers to a level of risk that is too small to be concerned with. Some refer to this as a "virtually safe" level.
Under U.S. tax rules, the de minimis rule governs the treatment of small amounts of market discount. Under the rule, if a bond is purchased with a small amount of market discount (an amount less than 0.25% of the face value of a bond times the number of complete years between the bond’s acquisition date and its maturity date) the market discount is considered to be zero. If the market discount is less than the de minimis amount, the discount on the bond is generally treated as a capital gain upon disposition or redemption rather than as ordinary income.
Under IRS guidelines, the de minimis rule can also apply to any benefit, property or service provided to an employee that has so little value that reporting for it would be unreasonable or administratively impracticable. Cash is not excludable, regardless of the amount.
Under European Community Competition Law some agreements infringing Article 81(1) of the EC Treaty are considered to be "de minimis" and therefore accepted. Horizontal agreement, that is one between competitors, will usually be de minimis where the parties’ market share is 10% or less, and a vertical agreement, between undertakings operating at different levels of the market, where it is 15% or less.