microscopic

adj. tiny, can only be seen with a microscope; belonging to a microscope (Optics)

A microscope (Greek: (micron) = small + (skopein) = to look at) is an instrument for viewing objects that are too small to be seen by the naked or unaided eye. The science of investigating small objects using such an instrument is called microscopy. The term microscopic means minute or very small, not visible with the eye unless aided by a microscope. The microscopes used in schools and homes trace their history back almost 400 years.
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Adjective
1. of or relating to or used in microscopy; "microscopic analysis"; "microscopical examination"
(synonym) microscopical
(pertainym) microscopy
2. too small to be seen except under a microscope
(synonym) microscopical, small
(antonym) macroscopic, macroscopical, large
(similar) atomic, atomlike, minute
3. extremely precise with great attention to details; "examined it with microscopic care"
(similar) precise
4. infinitely or immeasurably small; "two minute whiplike threads of protoplasm"; "reduced to a microscopic scale"
(synonym) infinitesimal, minute
(similar) small, little

(a.)
Alt. of Microscopical
  

So small it cannot be seen without the aid of microscope. As opposed to macroscopic (large enough to be seen with naked eye). A tiny tumor is microscopic while a big tumor is macrocopic.