the “normal” naturally-occurring version of something (protein, gene, bacterial strain, etc.) (as compared to a mutated version of it and/or something derived from it). So, for example, if you’re studying some protein and you make different versions (aka constructs) with different amino acid (protein letter) changes because you want to see what those amino acids do, the wild-type is the version you start with and then the changed ones are “mutants.” It can also be used to refer to the most common version of a gene in nature or the strain of some bacterium, etc. that you start with. Often abbreviated WT or wt.