I could compile a much more comprehensive dictionary, but these are what I deemed to be the most essential to musicians of all instruments.

2/4--Time signature meaning that there are two beats in each measure, each lasting one quarter note in length

2/8--Time signature meaning that there are two beats in each measure, each lasting one eigth note.

4/4--Time signature meaning that there are four beats in each measure, each lasting one quarter note.

6/8--Time signature meaning that there are six beats in each measure, each lasting one eighth note.

7/8--Time signature meaning that there are seven beats in each measure, each lasting one eighth note.

12/8--Time signature meaning that there are twelve beats in each measure, each lasting one eight note.

Allegro--A tempo marking meaning "cheerful."

Andante--"Walking tempo."  Relatively slow.

Cadence--A stylised closing in music that divides the piece or brings it to a conclusion.

Cadence, deceptive--A phrase that leaves the conclusion unresolved.

Cadenza--Unaccompanied ornamental solo passage, usually improvised and played by a soloist

Caesura--Break in the music, indicated by two diagonal tracks

Canon--A fugue in which each voice imitates the previous exactly

Coda--Ending passage that comes after a recapitulation.  Gives the piece closure.

Con Brio--With spirit

Con Fuoco--With fire

Crescendo--"Growing," increasing the volume of the music being played.

D.C.  (da capo)--From the top

Descant--An additional part to a piece that moves in parallel, oblique or contrary motion to the other voices.

Decrescendo/Diminuendo--Decreasing the volume at which a piece is being played.

Diminished chord--A chord that has the top note lowered one half-step.

Diminished interval--A perfect fourth, fifth or octave in which the top note is lowered by one half-step.

Dolce/Dolcissimo--Sweetly and as sweetly as possible, respectively.

Dolente--Sorrowfully, painfully

Dominant--The fifth note of a scale

 D.S. (dal segno)--From the sign

Dynamic--The loudness or softness that a piece should be played in.

Energico--Energetically

Enharmonic--Two separate notes stand for the same sound (i.e. F# and G-flat are the same thing)

Espressivo--Expressively

Etude--A study for a musician to develop technique

Fermata--This symbol indicates that the note should be held for a longer duration than its note value.

Fourth, perfect--An interval of four notes

Fifth, perfect--An interval of five notes

Forte (f)--Loud dynamic marking

Fortissimo (ff)--Louder than Forte

Grave--The slowest tempo in music

Half note--A note that lasts half the duration of a whole note

Harmonic Minor Scale--A minor scale in which the sixth and seventh notes are raised a half step on the ascending scale and lowered on the descending.

Intrada--The opening movement of a suite

Jubiloso--Jubilantly

Key signature--The marking of sharps or flats at the beginning of a piece or passage that indicates what key a piece should be played in

Lento--A very slow or solemn tempo

Legato--Meant to be played in a smooth, connected manner

Leggiero--Played in a light and brisk manner

Mediant--The third note of a scale

Melodic Minor Scale--A minor scale in which the seventh note of the scale is raised by a half-step.

Melody--In a composition with several voices, indicates the dominant tune of the piece

Menuet/Minuet--A slow dance in triple time

Mezzoforte (mf)--A fairly loud dynamic between Mezzopiano and Forte

Mezzopiano (mp)--Louder than Piano, but softer than Mezzoforte

Moll--German word for "minor"

Mosso--Movement, as in piu mosso (with movement)

Motif--A short tune that dominates and unifies a piece of music

Natural--A symbol meaning that the note should be played unaltered, without sharps or flats, after such a change has been made

Natural Minor Scale--A minor scale in which none of the notes are raised or lowered.

Ninth--An interval of nine notes

Non troppo--"Not too much," meaning that a piece should be played moderately

Obbligato--An accompanying part of the music that should not be omitted

Octave--Interval of eight notes

Off-beat--Any note played accented at a time other than on the beat

Parallel motion--Two voices moving in the same direction

Passionato--Meant to be played passionately

Pedal point/Organ point--A bass note that is played steadily while other voices move above it.

Piano (p)--Soft dynamic

Picardy third--A major chord at the end of a piece written in a minor mode.

Pietoso--Meant to be played tenderly

Precisamente--Played in a precise manner

Prelude--Instrumental piece that introduces a larger composition or set of compositions

Presto--The fastest tempo

Quarter note--A note that lasts the duration of one quarter of a whole note

Rhythm--The subdivision of a space/time into patterns of beats.

Ritardando--Gradually slowing the tempo of a piece

Ritenuto--Slowing a tempo more abruptly than a ritardando

Rubato--"robbed", taking part of one note's duration and giving it to another, stretching one note out. 

Sforzando--A sudden emphasis on a specific note.

Seventh--Interval of seven notes

Sixth--Interval of six notes

Sixteenth note--Note lasting one-sixteenth the duration of a whole note

Smorzando--Suddenly dying away

Subdominant--The fourth note of a scale

Submediant/Superdominant--The sixth note of a scale

Subtonic/Leading tone--The seventh note of a scale

Supertonic--The second note of a scale

Tempo--The speed of a performance of a composition.

Tempo Primo--Indicates that the performer(s) should resume the original tempo after a change in the tempo.

Tenuto--Indicates to hold a note longer than its intended duration

Thirty-second note--Note lasting one thirty-secondth the duration of a whole note

Time signature--Indicates the number of beats in each measure and the value of each beat

Tonic--The first note of a scale

Tritone--An interval of a diminished fifth

Un Peu--A little

Unison--Two voices/instruments playing the same exact note.

Vivace--"Lively" tempo.

Whole note--A note that lasts four beats.