Deci- (symbol d) is a decimal unit prefix in the metric system denoting a factor of one tenth. Proposed in 1793 and adopted in 1795, the prefix comes from the Latin , meaning "tenth". Since 1960, the prefix is part of the International System of Units (SI). Its most frequent use, however, is in a non-SI unit, the decibel, used to measure sound intensity (relative to a reference) and many other ratios.

Deci- (symbol d) is a decimal unit prefix in the metric system denoting a factor of one tenth. Proposed in 1793 and adopted in 1795, the prefix comes from the Latin , meaning "tenth". Since 1960, the prefix is part of the International System of Units (SI). Its most frequent use, however, is in a non-SI unit, the decibel, used to measure sound intensity (relative to a reference) and many other ratios.