There are some semi-strict definitions. From my favorite go-to ,
the ripresa, ritornello, and passacaglia are based on the 16th-century
Italian dance form. The ripresa or ritornello (often appearing as V-I
or IV-V-I) is a unit of music that precedes, follows, or alternates
with a dance. The internal ripresa could be used as a portion within a
dance or as a conclusion. While the number of internal riprese varies
according to the time elapsing between sections of a piece, its
harmonic design (i.e., the basic V-I pattern) is fixed. The concluding
ripresa, on the other hand, occurs at the end of a piece and shows a
greater harmonic variety through the insertion or substitution of
alternate chords. In the concluding ripresa, the basic V-I pattern
could be varied through the insertion, reshuffling, and mixing of
chords, resulting in unpredictable chains of chord progressions such
as IV-V-I-I, V-V-I-IV, V-V-I-II, or V-V-I-I. During the 17th-century,
these concluding riprese became independent sets and took the name of
the passacaglia or ciaccona. The technique of the passacaglia or
ciaccona then, is simply an ostinato of derived formulas of the
ripresa. Thus, the ripresa, ritornello, and passacaglia evolved from
the same harmonic pattern which originally functioned as a unit of the
Italian dance form
And for 'rondo,'
an instrumental form in which the first or main section (sometimes
called the 'refrain) is repeated between subsidiary sections (called
'episodes', 'couplets', 'digressions' or 'subordinate themes') and to
conclude the piece - usually in a lively tempo
Basically, a rondo is a more general form, but the ritornello could be pretty much a subset.