agnostic is one who holds that the existence of God is unknown and probably unknowable — and, in a weaker general sense, anyone unwilling to commit to a position on a question.
Thomas Henry Huxley coined the term in 1869, building it from the Greek agnōstos, "unknown" or "unknowable," to name his own stance against the theists and the atheists arguing around him. His claim was epistemological rather than theological: not that there is no God, but that the question sits outside what evidence can settle. The distinction still does real work, since atheism answers the question while agnosticism declines to.
The general sense arrived later and has largely overtaken the original in ordinary speech, describing anyone noncommittal on a disputed matter. Technology carried it further still. A system described as platform-agnostic or device-agnostic is not undecided about anything; it is indifferent by design, behaving the same regardless of what it runs on. That usage inverts Huxley's — his agnostic could not know, while an agnostic system does not need to.