An adverb expressing that something is likely to happen; a hedged prediction (often used for negative outcomes).
恐らく明日の会議は中止になるだろう。
The meeting will probably be canceled tomorrow.
彼は恐らく遅れると思うよ。道が混んでいるから。
I suspect he'll be late — the roads are congested.
恐らくそれは最善の方法ではないかもしれないが、一度試してみよう。
It may not be the best method, but we should probably try it once.