imprisonment, n. incarceration, durance, immurement, duress, [confinement], commitment.
impromptu, a. improvised, [offhand], extempore, extemporaneous.
improper, a. indecorous, [unseemly], [indecent], [inappropriate], [wrong], solecistic.
impropriety, n. unsuitableness, [unfitness], indecorum, [barbarism], [solecism].
improve, v. amend, ameliorate, [rectify], [better], [mend]; [utilize], [use]; [gain], get [better].
improvement, n. [betterment], amelioration; meliorism; utilization.
improving, a. ameliorative, [beneficial], amending.
impudence, n. [insolence], impertinence, effrontery, rudeness.
impudent, a. [insolent], [impertinent], [flippant], [pert], brazen, [saucy].
impulse, n. [motive], [incentive]; impulsion, [impetus].