na-adjective, noun
Almost always used in negative or interrogative contexts (e.g., 定かでない, 定かではない) to express uncertainty or lack of clarity. Rarely used affirmatively in modern Japanese.
彼の居場所は定かではない。
His whereabouts are not certain.
その噂が本当かどうかは定かでない。
It is not definite whether that rumor is true.
確か is more common and versatile, used both affirmatively and negatively, while 定か is almost exclusively used in negative or uncertain contexts.
明らか means 'clear' or 'obvious' and is often used affirmatively, whereas 定か emphasizes definiteness and is typically used negatively.
Derived from classical Japanese adjective 定かなり (sadakanari), meaning 'certain' or 'definite'. The modern form is almost exclusively used in negative constructions.