Homotonal (same-tonality) is a technical musical term pertaining to the tonal structure of multi-movement compositions. It was introduced into musicology by Hans Keller. According to Keller's definition and usage, a multi-movement composition is 'homotonal' if all of its movements have the same tonic (keynote).
'Homotonality' is by no means uncommon in compositions of the Baroque era: many Baroque multi-movement works based on dance-forms manifest the same tonic—and even the same mode (major or minor) – throughout. Thus, for example, J.S. Bach's solo violin partita BWV 1004 is homotonal [all movements in D minor], as is his solo flute partita BWV 1013 [all movements in A minor]. Similarly, Vivaldi's sonata for oboe and continuo RV53 (n.d.) is homotonal [all movements in C minor]. Homotonality is even encountered in some Baroque concertos: examples include Vivaldi's Cello Concertos RV401 (n.d.) [all movements in C minor] and RV416 (n.d.) [all movements in G minor], and Jean-Marie Leclair's Violin Concerto Op.7 No 1 (1737) [all movements in D minor].
With the Classical era, however, the situation changes. Outside of two-movement works (which, classically speaking, will maintain the same tonic for both movements and will thus be homotonal by definition), classical-era homotonality is relatively rare: a classical work in three movements will normally move to a different tonic for its middle movement, and a classical work in four movements will normally have at least one of its middle movements in a key other than the original tonic.
The classical composer most closely associated with the homotonal principle is Joseph Haydn.
Keller himself was keen to emphasise that different classical composers showed differing degrees of interest in homotonal structure:
Although Mozart, as opposed to Haydn, tended to work within narrow tonal frameworks, he did not carry the homotonal approach very far into his maturity . . . whereas Haydn did: some of the older master's greatest string quartets adhere to a single tonality"
"[U]nlike the mature Haydn, Mozart never came to write four movements without changing the keynote" [not actually true, but indeed it was much rarer for Mozart than Haydn once Mozart reached maturity]
Just because Haydn is more adventurous in his excursions into remote keys than Mozart, he sometimes needs a rigid tonal framework in order to contain them; unlike Mozart and like Haydn, whose developmental modulatory creative character he produced twinlike, Beethoven in his turn was to indulge in passionate tonal and harmonic contrast within homotonal frameworks.
Keller's coinage and concept have not become standard among musicologists. Musicologist William Drabkin, for example, asked the question "doesn't 'homotonality' sound a trifle queer?" 
The term 'homotonality' (referring to the manifest retention of a tonic) should not be confused with 'monotonality' (the theoretical position according to which a tonal structure has only one 'real' tonic, and all modulation is superficial or illusory).
Examples of 'homotonal' works (in more than two movements) from the classical era and afterwards are: