| imperceptible(adjective, hard, knowledge, faint, subtle) |
| indistinct(adjective, obscure, ambiguous, imperceptible) |
| invisible(adjective, knowledge, obscure, dark, subtle) |
| indistinguishable(adjective, knowledge) |
| faint(adjective, unclear, subtle, hazy, imperceptible) |
| hidden(adjective, secret, unseen, invisible) |
| insensible(adjective, knowledge) |
| vague(adjective, imperceptible) |
| impalpable(adjective, knowledge, unsubstantial, intangible) |
| inappreciable(adjective, knowledge, imperceptible) |
| obscure(adjective, solid, dark, unknown, dense) |
| undetectable(adjective, subtle, indistinguishable, imperceptible, unremarkable) |
| unnoticeable(adjective, knowledge, subtle, imperceptible, unremarkable) |
| unobservable(adjective, knowledge, imperceptible, invisible) |
| blurred |
| camouflaged(adjective, invisible) |
| concealed(adjective, invisible) |
| ill-defined |
| intangible(adjective, knowledge) |
| latent(adjective, obscure, dark, secret, unknown) |