Since the Baroque era, composers across Europe have mostly written performance instructions in Italian, regardless of their own nationality - a convention that's stuck ever since. These are the terms you'll run into most often, grouped by what they describe, followed by the general vocabulary - words like movement, opus, and cadence - that turns up in program notes and on this site. To decode a full marking like allegro ma non troppo word by word, with approximate speeds in BPM, try the tempo and terms translator. For words like aria, libretto, and bel canto, see the glossary of opera terms.

Tempo (speed)

Largo
Very slow and broad.
Adagio
Slow, at ease.
Andante
At a walking pace.
Moderato
A moderate, middle-of-the-road tempo.
Allegro
Fast and lively - one of the most common tempo markings in the entire repertoire.
Vivace
Lively and brisk, typically a shade faster than allegro.
Presto
Very fast.
Accelerando
Gradually speeding up.
Ritardando (rit.)
Gradually slowing down.
A tempo
Return to the previous, original tempo after a deviation.

Dynamics (volume)

Pianissimo (pp)
Very soft.
Piano (p)
Soft.
Mezzo piano (mp)
Moderately soft.
Mezzo forte (mf)
Moderately loud.
Forte (f)
Loud.
Fortissimo (ff)
Very loud.
Crescendo (cresc.)
Gradually getting louder.
Diminuendo / Decrescendo (dim. / decresc.)
Gradually getting softer.

Our dynamics and tempo markings guide goes deeper on how these two categories actually work in practice.

Articulation & expression

Legato
Smoothly connected, with no gap between notes.
Staccato
Short and detached, each note clearly separated.
Fermata
Hold the note or rest longer than its written value, at the performer's discretion.
Da capo (D.C.)
Return to the beginning of the piece.
Dal segno (D.S.)
Return to an earlier marked "sign" in the score, rather than the very beginning.
Cadenza
A soloist plays alone, often improvisatory in feel - most familiar from the concerto.
Con brio
With vigor and spirit.
Dolce
Sweetly, gently.
Espressivo
Expressively, with emotional emphasis.

General musical terms

Aria
A solo song in an opera, oratorio, or cantata, usually a moment of reflection or strong emotion. See opera.
Arpeggio
The notes of a chord played one after another rather than together - "broken" chords.
Cadence
A chord progression that ends a phrase or a piece, like a musical punctuation mark.
Chord
Three or more notes sounded at the same time.
Chromatic
Using notes outside a piece's home scale - the "in-between" notes that add color or tension.
Coda
"Tail": a closing section added after the main body of a movement.
Consonance / dissonance
Combinations of notes that sound stable and at rest (consonant) or tense and in need of resolution (dissonant).
Counterpoint
The art of combining independent melodic lines so they fit together. See fugue.
Glissando
A slide from one note to another, sweeping through the notes in between.
Harmony
The way notes sound together, and how chords move from one to the next.
Interval
The distance between two notes; an octave is the interval between one note and the next note of the same name.
Key
The home scale and home note a piece is built around, such as C major or A minor.
Major / minor
The two main types of scale and key - often heard, very roughly, as brighter and darker.
Melody
A sequence of single notes heard as a tune.
Modulation
Moving from one key to another within a piece.
Motif
A short, distinctive musical idea - like the four-note opening of Beethoven's Fifth - that a composer develops.
Movement
A self-contained section of a larger work, such as a symphony or concerto. Audiences usually wait until the last movement to applaud.
Opus (Op.)
"Work": a number given to a composition, usually in order of publication. Some composers' works use catalog numbers instead, like Mozart's K. or Bach's BWV.
Ostinato
A short pattern repeated persistently, like the snare-drum rhythm of Ravel's Boléro.
Polyphony
Music made of several independent melodic lines sounding together; monophony is a single line, as in chant.
Recitative
Speech-like singing that moves the story along between arias.
Scale
A series of notes in rising or falling order that forms the basic material of a key.
Score
The written music for a whole ensemble, showing every part at once.
Theme
A main melody that a piece is built on, often returning and transforming.
Timbre
Tone color - what makes a flute and a violin sound different even on the same note.
Tonality
The system of keys and home notes that organizes most Western music; music without it is atonal.
Trill
A rapid alternation between two neighboring notes, used as decoration.
Tutti
"All": everyone plays, as opposed to a solo.
Vibrato
A slight, regular wavering of pitch that gives sustained notes warmth.
Virtuoso
A performer of exceptional technical skill.

For the Italian markings in full - every common tempo word with its approximate speed, plus dozens more terms and abbreviations - see the tempo and terms translator.