Working hard at something over a long time without ever seeming to get tired or give up. It describes the kind of person who is still going long after everyone else has stopped, and it is almost always a compliment — it praises determination, not just energy. It attaches especially to work done for a cause: a tireless campaigner, a tireless defender of human rights, her tireless work for the charity. It is also used of the effort itself: tireless efforts, tireless dedication. Notice that it does not literally claim the person never feels tired — of course they do. What it means is that their tiredness never stops them, which is why it sounds like praise rather than a medical description. The related adverb is "tirelessly" ("she worked tirelessly for years"), and the noun is "tirelessness", which is rarer.