Industry job title whose meaning is ill-defined. Paweł Pokrywka argues it conflates technological expertise (senior Ruby/Rails/React/ClojureScript), computer-science knowledge, domain knowledge, and soft skills, and breaks down as soon as someone moves between languages or problem domains. Further issues: no clear authority to name someone senior (peers, HR, oneself), 'local seniors' in mediocre companies with six months of experience, imposter syndrome keeping qualified people from claiming it, and companies using titles instead of raises. Points to deeper questions about the industry's vocabulary, a given company's values, and the title-holder's own goals.