Opus infinitum (literally translated in Latin as "infinite work") is a term used by Rene Descartes to describe geometry in his landmark book of the same name.
In the context, Descartes was referring to the Sisyphean quest for knowledge - a work without end (i.e. infinite.) To Descartes, geometry (and more generally math) could never be completely known. 400 years after his work, we are still proving him right in the field.
Of course, the term can also apply to any other field of knowledge, particularly abstract fields with vast implications throughout other fields. We may someday master the entirety of the biology of a single fruit fly, but biology itself will remain an opus infinitum.