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An encyclopedia entry may begin with circa in the date of birth, for example: "Genghis Khan (c. 1162 – August 18, 1227)". The "c." means that he was born in about 1162, but the exact date is not certain. Sometimes the circa symbol is italicized to show that it is not in the English language.
Estimated values of different units may also begin with circa in research reports and scientific articles, for example "it involved c.450 kg of carved ivory". The "c." means that it was an estimated value based on an average value or any other coefficient that the researcher will generally define in the methodology.