Improvement, n.
1. The act of improving; advancement or growth; promotion in desirable qualities; progress toward what is better; melioration; as, the improvement of the mind, of land, roads, etc. I look upon your city as the best place of improvement. South. Exercise is the chief source of improvement in all our faculties. Blair.
2. The act of making profitable use or applicaton of anything, or the state of being profitably employed; a turning to good account; practical application, as of a doctrine, principle, or theory, stated in a discourse. "A good improvement of his reason." S. Clarke. I shall make some improvement of this doctrine. Tillotson.
3. The state of being improved; betterment; advance; also, that which is improved; as, the new edition is an improvement on the old. The parts of Sinon, Camilla, and some few others, are improvements on the Greek poet. Addison.
4. Increase; growth; progress; advance. There is a design of publishing the history of architecture, with its several improvements and decays. Addison. Those vices which more particularly receive improvement by prosperity. South.
4 more senses
5. pl.
Valuable additions or betterments, as buildings, clearings, drains, fences, etc., on premises.
6. (Patent Laws)
A useful addition to, or modification of, a machine, manufacture, or composition. Kent.
Shades of meaning
Fernald has no entry of its own for improvement; he lists it under profit, progress.
Where he writes about it:
In a thing good in itself all advance or progress is improvement; there is a growing tendency to restrict the words to this favorable sense, using increase indifferently of good or evil; one may say without limitation, "I am an advocate of progress." — from progress