technosignature
A biosignature looks for any life at all — microbes, plants, anything alive. A technosignature aims higher: it looks for signs of technology, evidence not just of life but of a civilization able to build things and reshape its world. It is any observable mark that intelligent, tool-using life might leave on the cosmos, from a deliberate radio broadcast to the faint pollution of an industrialized planet's air.
The most famous technosignature is a radio or laser signal — a narrow, artificial transmission unlike anything nature makes, which is exactly what SETI listens for. But technosignatures need not be messages sent on purpose. They could be the unintended leakage of a civilization's communications, industrial gases in a planet's atmosphere (like certain manufactured chemicals that have no natural source), the waste heat of vast energy use glowing in infrared, or hypothetical megastructures built to capture a star's light. The unifying idea is that technology might do things to a planet or star that biology and geology alone never would.
Technosignatures matter because they would answer not just 'is there life?' but 'are we alone as a thinking species?' — and because, unlike subtle gas imbalances, an unmistakable artificial signal could be far less ambiguous than a biosignature. The catch is that we have no idea how common, long-lived, or detectable technological civilizations are, so the search is a gamble with deeply uncertain odds. Decades of SETI have so far turned up nothing confirmed — a real result that bounds, but does not settle, how rare such civilizations might be.
Earth itself is a leaky technosignature: for a century our radio and television broadcasts have spilled into space, and our atmosphere carries traces of manufactured chemicals with no natural origin. A distant observer with the right instruments might read those as marks of technology — exactly what we are now learning to look for elsewhere.
A technosignature is the mark of a civilization, not just of life — and we make several.
We have no idea how common or long-lived technological civilizations are, so a non-detection so far neither proves nor disproves their existence — it only sets loose limits.