I've played in many bands and I do a lot of music production and the term "pad" is thrown around. My definition of a pad is something that is used to fill in the space of a piece usually with chords. However, I'm not sure if this is the official definition of a pad. I consider instruments like organs, strings and various synths to be pads. Perhaps it would help if the etymology of the word "pad" was known. I've checked several websites, but the answers to these questions are inconclusive.
My definition of a pad is something that is used to fill in the space of a piece usually with chords.
This is pretty much spot on. Like the orchestra uses the string section to play chords, people use synth pads to fill in with chords. I'll borrow the wiki definition:
A synth pad is a sustained chord or tone generated by a synthesizer, often employed for background harmony and atmosphere in much the same fashion that a string section is often used in orchestral music and film scores.
But, these kind of pads can also produce sounds; not just chords. Like for instance:
As you say, it's something that 'pads out' the mix to fill space.
Often the word 'pad' is used when talking about synthesized sounds, and refers to how a specific sound 'sits in the mix'. So you wouldn't really say that 'organs are pads' in general, because there are a lot of different organ sounds, some of which are very cutting upfront sounds (hence not 'pads'). But you could talk about an organ pad - a backgroundy, washy sound that had organ-like characteristics.
To some extent, a sound can be a pad because of how it's treated. Even a lead guitar sound could become a 'pad' if slathered in harmoniser, reverb, and delay.
The definition and history allows two derivation and associations that are covering our sensation when we are using the term pad and are congruent with the answers we have already got:
So I try and additional answer:
A pad in music can be for instance a string pad or synth pad.
and now the association: something soft (soft pad) to lay on! The melody lays on the pad of the chords - (long) held chords.
*1550s, "bundle of straw to lie on," a word of obscure origin (perhaps a merger of several separate words), possibly from or related to Low German or obsolete Flemish pad "sole of the foot," which is perhaps from PIE pent- "to tread, go" (see find (v.)), but see path (n.).
Sense of "soft cushion" is from 1560s, originally a soft saddle. Generalized sense of "something soft" is from c. 1700. Meaning "cushion-like part on the sole of an animal foot" in English is from 1790. The sense of "a number of sheets fastened or glued together at the edge" (in writing-pad, drawing-pad, etc.) is from 1865.
for more information also see:
It cites the same wiki quotation we have already got. At first it cites the same wiki quotation we have already got.
The interesting point is the definition asking:
What is the difference between a synth pad and a synth lead?
A good way to fully understand the definition of a pad is to contrast it with the type of synth sound at the opposite end of the spectrum. This is known as a synth lead.
Whereas a pad fills the space in the background of a track. The lead is very much in the foreground of the mix.
But now again my own sensation:
A pad is the pillow or a carpet, the underlayment of chords (by strings or synths) for the melody.