A rapid shimmer in a performance may be a trill, a tremolo or another effect. The words describe different musical instructions, and their meaning depends partly on the instrument and context.
In ordinary pitched notation, a trill alternates a written note with its neighbouring note, usually the one above. Key signature, accidentals and performance practice determine the actual pitches and how the ornament begins or ends. The abbreviation tr, often followed by a wavy line, is a familiar sign.
Tremolo is broader. It can mean rapid repetition of one note or chord, or alternation between notated notes or chords. Slashes through stems or between notes help specify it. The Essential Dictionary of Music Notation, hosted by MuseScore, illustrates these notational distinctions.
Listen for what is changing
Try a simple, original listening exercise at a keyboard. Play C and D alternately at a comfortable slow pace. Then repeat C alone. Finally alternate a C-major chord with a different comfortable chord. The exercise is not a performance rule; it separates the changing pitch material from the repeated action.
Increase speed only while the movement remains relaxed. A teacher can help with technique and style. The fastest possible repetition is not automatically the most convincing musical result.
Now listen to a short passage while following its score. Ask whether the performer repeats one written sound, alternates neighbouring pitches or moves between a wider set of notes. Note the instrument, because a string player’s bowing action and a pianist’s key action create different textures.
The word also appears on electronic effects. An amplifier’s tremolo commonly varies loudness over time; that is a different context from a two-note tremolo in a score. A device manual should explain what its labelled control actually changes.
When discussing a passage with another musician, point to the notation or give a timestamp. “The trembling sound” leaves several possibilities open. Naming the notes, the rhythm and the instrument makes the question much easier to answer.