Clipping
Clipping is a digital phenomenon where any audio that surpasses 0dB on a digital meter is lost data. Clipping usually creates unpleasant sounds and can be damaging to speakers in extreme cases. While DAWs have ways to reduce the effect of clipping, if your audio in clipping in a final render, audio information has been lost. It's best practice to never have audio clip, unless its for sound design reasons.
Colour
Colour in an audio context will usually mean adding more saturation or harmonics to a sound, it can also include subtle noise. A lot of analog gear adds colour to incoming audio due to the saturation & noise inherent in physical gear. Colour is usually a very subtle form of saturation that would not be easily perceptible to a regular listener, but adds to the overall sound of a track.
Dynamic Range/ Dynamics
Dynamic Range refers to the variation in volume between the softest and loudest parts of a sound or song.
Envelope
An envelope is a shape of a sound over time. It includes the attack, hold, decay, sustain and release. An envelope is most used in sound design and synthesizers.
Headroom
Headroom refers to the space above the peaks in your signal. You should always have peaks below 0dB to prevent unwanted distortion. The space between your peak & 0dB is your headroom.
In-the-box
Working on a song using primarily digital tools and effects on a computer. This usually means a completely digital workflow with a DAW, virtual synths & virtual effects. While some people believe in-the-box mixing is not comparable to out-the-box, as technology progresses, the differences have decreased considerably.
Out-the-box
Working on a song using outboard gear like tape machines, analog effect units, mixing desks and many other tools that you would find in professional recording studios. An out-the-box workflow is much harder to achieve simply due to the cost and difficulty of setting everything up.
Semitone
It is a music theory concept. A semitone is an interval, the smallest step between notes. If you move from one note to the first adjacent note, that is a semitone interval.
Sound Stage
The overall width & depth of the music you are listening to. A wider sound stage means the sounds are further a part between the left and right speakers, while conversely, a small sound stage will sound very narrow with more sound in the middle of the stereo image.
Stereo Image
The stereo image of a sound/ track refers to the relation between the left and right speakers and where a sound is located between the two speakers. This is usually acheived with panning to control how much audio is being played through one speaker as opposed to the other to give a sense of the sound coming from a certain direction.
Timbre
Timbre (pronounced /'tæmbər/ -- Tam-Ber) is a way to describe the entire feel of a sound. The tone, pitch, warmth, feel, of a sound. Timbre is a catch all term to differenciate sounds from each other.