Nebulous
Etymology
From Latin "nebula," "mist, cloud, vapor." The literal astronomical sense — a hazy patch of light, a nebula — came first in English. From that cloudiness grew the figurative meaning: a concept as blurred and hard to grasp as fog.
adjective · /NEB-yuh-lus/
Vague and ill-defined; lacking clear shape or outline.
What it names
Use it for ideas or plans that never resolve into anything definite — a nebulous strategy, a nebulous feeling — where the trouble is not error but the absence of firm edges.
Example
Their plan for after graduation was nebulous at best — something about travelling, something about a startup.
Related idea
Compare tacit: the tacit thing is clear but deliberately unspoken, while the nebulous thing may be spoken at length and still refuse to come into focus.