Antigen
Any molecule the immune system can recognize and react to — the general category of which a neoantigen is the tumor-specific, mutation-derived special case.
An antigen is anything an immune receptor can bind: a viral protein, a bacterial sugar, or a fragment of one of the body's own proteins. Most self-antigens are tolerated (ignored) because the immune system was trained not to attack them.
Tumor antigens split into two camps: tumor-associated antigens (normal proteins the tumor over-expresses — shared across patients but risky to target because they also appear on healthy tissue) and neoantigens (created by the tumor's own mutations — patient-specific and truly foreign). The field's bet on neoantigens is precisely that being non-self makes them safer and more immunogenic targets.