that cannot be seen; very slight or gradual

hard to sense; faint

of which the effect cannot at that point be judged as important or not, or cannot be estimated

imperceptible

too small or slight to be noticed

imperceptible

that cannot be known because of the senses or fully understood, but can be sensed by the feelings

indefinite, obscured

that cannot be seen

unable to be seen; hidden

based on thought and reasoning; (of poetry) intellectual and psychological, usually about themes such as love, religion and death

not physical; without physical presence

imaginary; not real

fake, make-believe; hypothetical

not seen

hidden

not clear or distinct; slight; (of a person, expression, behaviour) uncertain, especially about details

not definite or clear