Improvise, v. i. — Webster enters it under “improvise”
To produce or render extemporaneous compositions, especially in verse or in music, without previous preparation; hence, to do anything offhand.
1 other Webster entry under this spelling
Improvise, v. t. [imp. & p. p. Improvised; p. pr. & vb. n. Improvising.]
1. To compose, recite, or sing extemporaneously, especially in verse; to extemporize; also, to play upon an instrument, or to act, extemporaneously.
2. To bring about, arrange, or make, on a sudden, or without previous preparation. Charles attempted to improvise a peace. Motley.
3. To invent, or provide, offhand, or on the spur of the moment; as, he improvised a hammer out of a stone.
Shades of meaning
Fernald has no entry of its own for improvised; he lists it under extemporaneous.
He lists it without comment; the entries above explain its neighbors.