"Strong" means that the evidence is reported as credible by mainstream press and/or is accepted by at least a large minority of relevant scientists (e.g. 1 in 3).

"Evidence" can be direct (e.g. the physical recovery of alien bacteria from Mars) or indirect (e.g. telescopic observations of technosignatures). Evidence can also be of the past existence of aliens, such as fossils.

"Aliens" are defined liberally. This market counts anything that is arguably non-earth-originating life as aliens. This includes both intelligent and non-intelligent beings, non-carbon-based biology, and things, such as viruses, for which there is controversy over their status as life.

If there is uncertainty in applying the resolution criteria, I will resolve the market using reasonable judgement.