thumb|Sample of [[cobalt-60 that emits 1 μCi (microcurie) of radioactivity; i.e. 37,000 decays per second.]]
The curie (symbol Ci) is a non-SI unit of radioactivity originally defined in 1910. According to a notice in Nature at the time, it was to be named in honour of Pierre Curie, but was considered at least by some to be in honour of Marie Skłodowska-Curie as well, and is in later literature considered to be named for both.
It was originally defined as "the quantity or mass of radium emanation in equilibrium with one gram of radium (element)", after more accurate measurements of the activity of Ra (which has a specific activity of ).
In 1975 the General Conference on Weights and Measures gave the becquerel (Bq), defined as one nuclear decay per second, official status as the SI unit of activity.
Therefore:
: 1 Ci = = 37 GBq
and
: 1 Bq ≅ ≅ 27 pCi
While its continued use is discouraged by the National Institute of Standards and Technology (NIST) and other bodies, the curie is still widely used throughout government, industry and medicine in the United States and in other countries.
At the 1910 meeting, which originally defined the curie, it was proposed to make it equivalent to 10 nanograms of radium (a practical amount). But Marie Curie, after initially accepting this, changed her mind and insisted on one gram of radium. According to Bertram Boltwood, Marie Curie thought that "the use of the name 'curie' for so infinitesimally small [a] quantity of anything was altogether inappropriate".