If something is believable, you can believe it: it seems possible, likely, or real enough that the mind does not push it away. We use it for stories, excuses, acting, and explanations — a believable reason, a believable performance, a barely believable excuse. It is a calm everyday adjective, not a scientist's word, and it lives on a scale: just believable, not very believable, completely believable. You can say it of a person ('a believable witness') but English reaches for it more often with things people make up or perform. Though actually, speakers say unbelievable far more, and they stretch that one until it is almost a noise of feeling: 'unbelievable' can mean 'so good / so bad / so strange I can hardly take it in,' not only 'this cannot be true.' Believable stays nearer home. It still asks a quiet question: could this really be so?