deci

Deci is a decimal metric prefix used in the International System of Units (SI) to denote one tenth of a unit. The prefix name is "deci-" and its symbol is "d" (lowercase). It was adopted into the SI in 1960 as part of the systematization of metric prefixes. The prefix is derived from Latin decem, meaning ten, reflecting its role as one tenth.

Placed before a unit, deci- indicates 0.1 of that unit. Examples include decimeter (dm), which is 0.1 meter; deciliter (dL), 0.1 liter; decigram (dg), 0.1 gram; and decisecond (ds), 0.1 second. When combined with unit symbols, the prefix and the symbol are written without space, and the prefix symbol is always lowercase; unit symbols are case-sensitive (for instance L for liter, m for meter).

Deci is used across science, engineering, and commerce where measurements fall in the range between base units and smaller prefixes such as centi- (0.01) or milli- (0.001). It is part of the broader set of SI prefixes that expresses powers of ten.

Notes: The term deci can also appear in contexts outside of SI as a part of proper names or acronyms, but in metric usage it denotes one tenth of a unit.