many obstacles; innumerable difficulties
Refers to a great number of hardships or obstacles, often used in formal or literary contexts. Commonly appears in the set phrase 万難を排して (overcoming all difficulties).
He carried out the plan, overcoming all difficulties.
This project will involve innumerable difficulties.
Compound of 万 (ten thousand, myriad) and 難 (difficulty, hardship). The exact historical derivation is uncertain, but the combination literally means 'myriad difficulties'.