Undefinable (adjective)

1. Not able to be defined or described in a clear or definite manner.
2. Not capable of being clearly or definitely described or identified.

Origin:

Late 19th century: from un-1 + definable, on the pattern of words such as unanswerable.

Examples:

1. The concept of love is undefinable.
2. The term is undefinable and has no clear meaning.
3. The artist's style is undefinable and cannot be easily categorized.
4. The undefinable feeling of contentment filled him.
5. The undefinable nature of the universe is a subject of much debate among scientists.