In no particular order
KTS: Key Telephone System, includes KSU, phones, cable, accessories.
KSU: Key Service Unit, the control unit or "brain" that makes the system work. It holds circuit boards, power supply, accessories, punchdown block(s)
KTU: Key Telephone Unit, a printed circuit card or a circuit in another format that provides a specific function and goes inside a KSU.
BSP: Bell System Practices, the rules for doing just about anything within the Bell System. BSP books covered phone equipment, employee relations, driving trucks, first aid, erecting telephone poles, discovering wiretaps, everything.
KS: Kearney Sequence (or perhaps another word that begins with "S"), a numerical designation for a particular piece of hardware either made by Western Electric or bought for the Bell System from another manufacturer. There was a major Western Electric factory in Kearney, New Jersey.
ComCode: another parts numbering scheme. One part could have multiple part numbers.
USOC: Uniform Service Ordering Code, an alphanumeric designation for a specific service or device provided by the Bell System. USOCs have survived as designations for phone and data jacks.
KV: a key telephone designed for use on a desk or table, most commomnly a 6-button phone.
KVW: A key phone designed for wall-mounting.
K10: A 10-button key phone.
K20: A 20-button key phone.
K30: You should be able to figure it out.
CV: A single-line phone.
C stock: Refurbished phones