Latin for "by the court." An opinion from an appellate court that does not identify any specific judge who may have written the opinion.
A "per curiam" decision is a decision delivered via an opinion issued in the name of the Court rather than specific judges. Most decisions on the merits by the Supreme Court (and other appellate courts in the U.S.) take the form of one or more opinions signed by individual justices (and joined in by others). Even when such signed opinions are unanimous, they are not termed "per curiam." "Per curiam" decisions are given that label by the Court itself and tend to be short. Usually, though not always, they deal with issues the Court views as relatively non-controversial.
For examples, see, e.g., Wood v. Bartholomew, 516 U.S. 1 (1995) and Kimberlin v. Quinlin, 515 U.S. 321(1995). However, they are not necessarily unanimous. Indeed, some per curiam decisions are accompanied by dissenting opinions. See, e.g., Bush v. Gore, 531 US 98 (2000).
The top appellate courts of most states (including, e.g., Alabama, Arkansas, Connecticut, Florida, Georgia, Maryland, Nebraska, Nevada, New Mexico, North Carolina) use the same terminology.