I is for Intangible

intangible (adjective): something that exists but that cannot be touched, exactly described, or given an exact value. 

This may mean:

• something that is unable to be physically touched: (immaterial, incorporeal, impalpable, insubstantial, ethereal, ghostly)
• something that is difficult to perceive or grasp: (elusive, impalpable, imperceptible, vague.)
• something that is abstract or theoretical: (hypothetical)