A lifeform was not necessarily organic — which was to say carbon-based. Nevertheless, Federation scans for lifeforms often looked only for organic life. In at least one known case, this limited view of what constituted a lifeform had disastrous consequences for a Federation terraforming project. (TNG: "Home Soil")
There are incredible similarities between the lifeforms on different worlds in the Star Trekuniverse. A probable explanation for this may be the existence of the ancient humanoidsDNA during evolution.