An order in architecture is a certain assemblage of parts subject to uniform established proportions, regulated by the office that each part has to perform. Coming down to the present from Ancient Greek and Ancient Roman civilization, the architectural orders are the styles of classical architecture, each distinguished by its proportions and characteristic profiles and details, and most readily recognizable by the type of column employed. The three orders of architecture—the Doric, Ionic, and Corinthian—originated in Greece. To these the Romans added, in practice if not in name, the Tuscan, which they made simpler than Doric, and the Composite, which was more ornamental than the Corinthian. The architectural order of a classical building is akin to the mode or key of classical music; the grammar or rhetoric of a written composition. It is established by certain modules like the intervals of music, and it raises certain expectations in an audience attuned to its language.

Whereas the orders were essentially structural in Ancient Greek architecture, which made little use of the arch until its late period, in Roman architecture where the arch was often dominant, the orders became increasingly decorative elements except in porticos and similar uses. Columns shrank into half-columns emerging from walls or turned into pilasters. This treatment continued after the conscious and "correct" use of the orders, initially following exclusively Roman models, returned in the Italian Renaissance. Greek Revival architecture, inspired by increasing knowledge of Greek originals, returned to more authentic models, including ones from relatively early periods.

Each style has distinctive capitals at the top of columns and horizontal entablatures which it supports, while the rest of the building does not in itself vary between the orders. The column shaft and base also varies with the order, and is sometimes articulated with vertical concave grooves known as fluting. The shaft is wider at the bottom than at the top, because its entasis, beginning a third of the way up, imperceptibly makes the column slightly more slender at the top, although some Doric columns, especially early Greek ones, are visibly "flared", with straight profiles that narrow going up the shaft.

The capital rests on the shaft. It has a load-bearing function, which concentrates the weight of the entablature on the supportive column, but it primarily serves an aesthetic purpose. The necking is the continuation of the shaft, but is visually separated by one or many grooves. The echinus lies atop the necking. It is a circular block that bulges outwards towards the top to support the abacus, which is a square or shaped block that in turn supports the entablature. The entablature consists of three horizontal layers, all of which are visually separated from each other using moldings or bands. In Roman and post-Renaissance work, the entablature may be carried from column to column in the form of an arch that springs from the column that bears its weight, retaining its divisions and sculptural enrichment, if any. There are names for all the many parts of the orders.

The heights of columns are calculated in terms of a ratio between the diameter of the shaft at its base and the height of the column. A Doric column can be described as seven diameters high, an Ionic column as eight diameters high, and a Corinthian column nine diameters high, although the actual ratios used vary considerably in both ancient and revived examples, but still keeping to the trend of increasing slimness between the orders. Sometimes this is phrased as "lower diameters high", to establish which part of the shaft has been measured.

There are three distinct orders in Ancient Greek architecture: Doric, Ionic, and Corinthian. These three were adopted by the Romans, who modified their capitals. The Roman adoption of the Greek orders took place in the 1st century BC. The three ancient Greek orders have since been consistently used in European Neoclassical architecture.

Sometimes the Doric order is considered the earliest order, but there is no evidence to support this. Rather, the Doric and Ionic orders seem to have appeared at around the same time, the Ionic in eastern Greece and the Doric in the west and mainland.

Both the Doric and the Ionic order appear to have originated in wood. The Temple of Hera in Olympia is the oldest well-preserved temple of Doric architecture. It was built just after 600 BC. The Doric order later spread across Greece and into Sicily, where it was the chief order for monumental architecture for 800 years. Early Greeks were no doubt aware of the use of stone columns with bases and capitals in ancient Egyptian architecture, and that of other Near Eastern cultures, although there they were mostly used in interiors, rather than as a dominant feature of all or part of exteriors, in the Greek style.