Indefinable (adjective)

1. Impossible to define or describe precisely.
2. Impossible to identify or distinguish clearly.

Origin:

From in- + definable.

Examples:

1. The concept of love is often considered to be indefinable.
2. The meaning of the poem is indefinable.
3. The color of the sky at dusk is indefinable.
4. The indefinable feeling of happiness.
5. The indefinable essence of the painting.