Doubtless means certainly or without doubt. When you say something will doubtless happen, you're expressing strong confidence that it will occur. A teacher might say, “You've studied hard, so you will doubtless do well on the exam.”

The word carries a tone of complete certainty, though sometimes with a hint of understatement. When a scientist observes dark clouds gathering and says, “It will doubtless rain soon,” she's not really expressing doubt at all: she's quite sure it will rain. That's the interesting thing about doubtless. Despite having “doubt” in it, the word means the opposite: no doubt whatsoever.

You'll often see doubtless used when someone is making a prediction based on strong evidence or past experience. “This movie will doubtless be popular” means the speaker sees clear signs it will succeed. “He is doubtless the fastest runner on the team” means there's no question about it.

The word adds a slightly formal or thoughtful tone to speech and writing. Instead of simply saying “certainly” or “definitely,” using doubtless can suggest you've considered the matter carefully and reached a firm conclusion.