Phil Spector is noted for his distinctive "wall of sound" on the records that he produced in the early sixties. What is it and how did he achieve it?
Phil Spector has been quoted saying that the pinnacle of this technique is heard on his production of "River Deep Mountain High", by Tina Turner:
When you listen to that, notice how there are so many instruments playing. There is a bass part that is probably played by three bass players, a guitar part that again could be three guitarists, a strings section, a brass section, it sounds like two layers of backing vocals (one layer sort of like "ooohs" and another that is harmonizing with Tina), and three types of percussion - at least one drum kit, congas, and auxiliary percussion (e.g., shakers, etc.). Oh and there's some kind of bells or vibraphone/marimba, and probably one to three pianos getting lost in the mix.
Also, there is a decent amount of reverb on everything.
As noted in the comments, not only are there many parts being played at once, but also each part is played identically by more than one instrument in perfect unison and synchronization. In some cases (multiple bass guitars), that thickens the sound, and in other cases (say, a piano and marimba playing at the same time) it creates the illusion of a new kind of instrument that combines the qualities of the instruments playing the part.
One thing that was revolutionary about the concept is that it didn't matter if some instruments got lost in the mix - that was partly the point. He was trying to create a kind of pop orchestral sound, where the individual players were not as important as the whole sonic image.
In terms of how he achieved it, a lot of it was orchestration, but probably most of the hard work would fall into the category of what a producer does that no one else does. There wasn't the technology to record and mix 20+ instruments separately at the time, but that didn't seem like a lability to Spector. Getting the sound in the room was part of how records were produced, so he first worked to get a sound in the room that was different from other producers.
As mentioned above, he crammed a ton of musicians into one room and orchestrated the parts for layering and doubling. Then he worked the musicians for hours before even recording, to get the performance and mix in the room that he wanted. At the same time, there would be aspects of musicians placement, mic placement, and performance notes that would influence what each mic heard. The sounds would all be bouncing off the walls and mixing in the room and then entering different mics. Obviously the closest mic to a musician picked up mostly that musician, but it would also get a bit of everything else, and that musician would show up a little bit in every other mic.
Once the sound in the room was what he wanted, he would go back to the mixing booth and get the different sounds in the different mics all blended in the way that created the effects and sounds he was trying to capture. These two components of crafting the room sound and then crafting the mixed and recorded sound are really the heart of the job of music production (assisted by an engineer, who are sometimes half-producers themselves).
Over time, he was able to have the same session artists sit in the same place with the same microphones and more accurately and quickly predict and craft the sound that he wanted. Hence, The Wrecking Crew, AKA "Phil Spector's Wall Of Sound Orchestra".
Phil Spector is definitely the first arranger that comes to mind wrt 'wall of sound.' However it may be that John Coltrane's late 50s jazz was the origin of the term as this search in Google Books suggests https://books.google.com/books?id=kfFgNABSuuUC&pg=PA176&lpg=PA176&dq=coltrane+wall+of+sound&source=bl&ots=4LHD0u-q4d&sig=odxCu4N8iQ_JWPfh-XW6QmpVWKc&hl=en&sa=X&ved=0ahUKEwjWjaLV25zbAhXH7YMKHU6gC78Q6AEIcDAM#v=onepage&q=coltrane%20wall%20of%20sound&f=false
Quoting from p 176 of Thomas Larson's book History and Tradition of Jazz:
During Coltrane's stay with (Miles) Davis from December 1957 through May 1960, he became a star. Inspired by a spiritual awakening after kicking his drug habit, he became even more adventurous in his solos. He began to break free of the rhythmic boundaries of the music, creating a continuous wall of sound that critic Ira Gitler called "sheets of sound" in a late 1958 essay in Down Beat.
The first edition of Larson's book was published in 2002. Regardless it suggests that the original meaning and usage of wall of sound may have begun with Coltrane's jazz and the association with Spector's orchestration as a wall of sound was imitative.