Life is a self-sustaining chemical system capable of Darwinian evolution, with the ability to grow, reproduce, maintain homeostasis, and respond to stimuli, all using an enclosed cellular structure.

This is a modification of the definition given by NASA astrobiologist Gerald Joyce, who simply defines life as the first clause: "Life is a self-sustaining chemical system capable of Darwinian evolution" (though he claims it's not his own personal coinage). Unlike his definition, the above definition will exclude viruses and self-replicating robots. Claude wrote it at my request. It represents an attempt to condense the mainstream biology definition as tersely as possible.