Synonyms: These adjectives apply to what is capable of being apprehended as being real by the mind or through the senses. Perceptible is the least specific: a perceptible pause in the flow of his speech.
Palpable applies both to what is perceptible by means of the sense of touch and to what is readily perceived by the mind: "The advantages Mr. Falkland possessed . . . are palpable" (William Godwin).
What is appreciable is capable of being estimated or measured: dumping appreciable amounts of noxious waste into the harbor.
Noticeable means easily observed: noticeable shadows under your eyes.
Discernible means distinguishable, especially by the faculty of vision or the intellect: no discernible progress in the contract negotiations.