What are olfactory indicators?

Olfactory indicators are those indicators that help to identify whether the given solution is acidic or basic by changing their smell instead of colour as other indicators do. For example onion, vanilla, clove, etc
Onion: Paste or juice of onion loses its smell when added with base. It does not change its smell with acid.
Vanilla: The smell of vanilla vanishes with base, but it’s smell does not vanishes with an acid.