na-adjective, noun which may take the genitive case particle 'no', noun
extremely difficult; next to impossible
A relatively formal word for something so hard that it is almost impossible. Common in set-like phrases such as 至難の業 and 至難を極める.
Under these conditions, convincing everyone is extremely difficult.
Breaking his record is a nearly impossible task.
Persuading him is next to impossible.
Solving this difficult problem is extraordinarily hard.
不可能 flatly means 'impossible', whereas 至難 usually implies extreme difficulty rather than outright impossibility.
難しい is the everyday, plain adjective for 'difficult', while 至難 is a formal, intensified term for near-impossible situations.
Standard kanji spelling; this is the only written form, with a high-priority reading.
A Sino-Japanese compound written with 至, associated with reaching an utmost degree, and 難, difficulty. The spelling transparently conveys 'utmost difficulty.'