That would be a contractor or contingency worker for me. They are hired to do X. When they leave their work stops, a critical difference with a consultant.

Consultant does mean something. Means you temporarily hire experience in a field you are not familiar with to bring competency to your company, train/knowledge transfer to employees, and have a quick up ramping of a project which can continue when the consultant leaves because now you have a proper architecture and your employees are trained to use it, work with and extend it.