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A Biosignature, generally, is a measurable phenomenon that indicates the presence of life. The term biomarker is sometimes used as a synonym.
Biosignatures in Astrobiology
In Astrobiology, a biosignature is a sign of the presence of life in general and is usually studied with an eye towards identifying and detecting extraterrestrial life. Often, biosignatures in this context are chemical: the detection of DNA in the Martian soil or oxygen in an extrasolar planet's atmosphere would likely be interpreted as diagnostic evidence of life. (Oxygen is highly reactive with many minerals and metals, and is generally assumed to persist in an atmosphere only if lifeforms are actively producing it, as photosynthetic plants and cyanobacteria do on earth.
 In 1996 some researchers suggested that these microscopic structures on the ALH84001 meteorite were fossilized bacteria Biosignatures need not be chemical, however. The shape and size of certain objects may potentially indicate the presence of life. For example, tiny magnetite crystals in the Martian meteorite ALH84001 were the longest-debated of several potential biosignatures in that specimen because it was believed until recently that only bacteria could create crystals of their specific shape. Other potential biosignatures studied in the ALH84001 meteorite included putative nanofossils, tiny rocklike structures whose shape was a potential biosignature because it resembled known bacteria. Most scientists ultimately concluded were far too small to be fossilized cells.
From this point of view, even the hypothetical radio signatures that SETI scans for would be an electromagnetic biosignature, since a message from intelligent aliens would certainly demonstrate the existence of extraterrestrial life.
The Viking Missions to Mars
The Viking Missions to Mars in the 1970s conducted the only thorough experiments to date which were explicitly designed to look for biosignatures on another planet. Each of the two Viking landers carried three life-detection experiments which looked for signs of biochemical metabolism.
More details can be found in the entry on the Viking Biological Experiments.
Biosignatures in Geochemistry
In Geochemistry, Geobiochemistry, and Geomicrobiology, biosignatures are often used in a more precise fashion to determine which living organisms are or were present in a sample. For example, the particular fatty acids measured in a sample can indicate which types of bacteria and archaea live in that environment. When used in this sense, geochemists often prefer the term biomarker.
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