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Webster 1913 Edition

Adj.

Serving or tending to concentrate; characterized by concentration.

A discrimination is only possible by a

concentrative

act, or act of attention.

Sir W. Hamilton.

concentrative ‎(comparative more concentrative, superlative most concentrative)

Characterised by concentration.
- 1841, Edgar Allan Poe, "The Murders in the Rue Morgue":
  To observe attentively is to remember distinctly; and, so far, the concentrative chess-player will do very well at whist [...].