grey matter 
noun [mass noun]
1The darker tissue of the brain and spinal cord, consisting mainly of nerve cell bodies and branching dendrites. Compare with white matter.The more severe a patient's ADHD symptoms - as rated by parents and clinicians - the smaller were their frontal lobes, temporal gray matter, caudate nucleus and cerebellum....
- In some tissues, such as gray matter in the brain, water also diffuses in an approximately spherical pattern.
- They had less grey matter in the frontal lobes and had enlarged ventricles.
1.1 informal Intelligence: I wish I had a little of her grey matter...
- College professors like Felix Bethel, who have considerable intellectual grey matter do not hold these beliefs.
- They are still behaving as if they were the stupid party, even though they manifestly possess rather more grey matter, head for head, than Labour.
- He holds degrees in criminal justice and psychology and I have already been picking his scholarly gray matter.
Synonyms
intelligence, intellect, intellectual capacity, mental capacity, brains, brainpower, cleverness, wit, wits, powers of reasoning, reasoning, wisdom, sagacity, acumen, discernment, shrewdness, judgement, understanding, common sense, sense
informal nous, savvy, braininess, upper storey