Contemporaneous means happening or existing at the same time as something else. When historians study contemporaneous accounts of a battle, they're reading descriptions written by people who were actually there when it happened, not stories written years later. If two scientists make contemporaneous discoveries, they each figure out the same thing independently during the same period, like Alexander Graham Bell and Elisha Gray both developing telephone technology in the 1870s.

Contemporaneous evidence matters in courts because a diary entry written on the day something happened is usually more reliable than someone's memory years later. A letter written contemporaneously with an event captures what people thought and felt right then, before time could change or blur their recollections.

You might hear someone say that the Declaration of Independence and Adam Smith's The Wealth of Nations were contemporaneous works, both published in 1776. The word emphasizes that these things weren't just close in time but actually overlapped, existing or happening together in the same historical moment.