Cacophony
Discord, an unpleasant sonority.
What it means
Cacophony literally means bad sound and designates a sonority that is felt to be unpleasant, raw, or disordered. The word is a judgment, not a technical finding — it says something about the effect on the listener, not about the construction of the sound. Its counterpart is the euphonious, euphony.
Musical use
Historically the term is familiar above all as a term of combat: music that contradicted the listening habits of a time was regularly called this by its opponents. Almost every stylistic innovation has attracted this reproach, and in almost all cases getting used to it has later invalidated it. This is not a side note but shows how flexible the boundary between euphony and discord is.
To be distinguished from this is dissonance as a technical term: it designates a sonority that demands resolution and belongs to the necessary stock of music. Without dissonance there would be no tension and thus no resolution either; it is not a fault but a means. Likewise the cluster, a chord of closely spaced notes, is a deliberately used means of sound and not mere sharpness.
Where the term factually applies, it usually means something other than dissonance: an unbalanced texture, hidden voices, unclean intonation, an unfavourable spacing — chord tones set closely in a low register sound murky, however they are composed.
The term often serves as a pejorative judgment on music that is felt to be disordered and is therefore strongly dependent on the listener's standpoint. The criticism of the time has called many works cacophony that later counted as significant. In composition, deliberate confusion is occasionally used to depict noise, chaos, or satire.
Performance practice
If a sonority sounds bad, first check intonation, spacing, and balance before you blame the music — in most cases it is one of these three points. Do not take dissonances back but let them be audible and resolve them distinctly. And meet unfamiliar sounds with patience when studying; what seems harsh at the first pass often falls into order by the third.