A. A. Gill

This is the trouble with cheating: there are no acceptable rules, or laws. It could be a smile, or dancing to a song that you considered to be indefinably 'ours'. It can feel like cheating to go to a restaurant that you used to go to with someone else. Keeping photographs of exes can infuriate, like retrospective cheating.

British Writer

Use 'indefinably' when you're trying to express a quality that can't be pinned down or easily explained.

Describing Ambiguity

'Indefinably' suggests a nuance that's felt rather than concretely identified.

Subtle Nuance

Be mindful that what seems 'indefinably' unique may be culturally specific and clear to others.