[[File:PCIExpress.jpg|250px|thumb|Four [[PCI Express]] bus card slots (from top to second from bottom: ×4, ×16, ×1 and ×16), compared to a 32-bit [[conventional PCI]] bus card slot (very bottom)]]
In [[computer architecture]], a '''bus''' (historically also called a '''data highway'''<ref name="Hollingdale_1958">{{cite conference |conference=Applications of Computers, University of Nottingham 15–19 September 1958 |title=Session 14. Data Processing |author-first=Stuart H. |author-last=Hollingdale |date=1958-09-19 |url=https://www.chilton-computing.org.uk/acl/literature/othermanuals/nottingham/p014.htm}}</ref> or '''databus''') is a communication system that transfers [[Data (computing)|data]] between components inside a [[computer]] or between computers.<ref>{{cite book |url=https://books.google.com/books?id=YVi8HVN-APwC&q=computer+buss+-sam&pg=PA27 |title=What Every Engineer Should Know about Data Communications |first=Carl |last=Clifton |publisher=CRC Press |date=September 19, 1986 |page=27 |isbn=9780824775667 |quote=The internal computer bus is a parallel transmission scheme; within the computer.... |url-status=live |archiverchive-url=https://web.archive.org/web/20180117151300/https://books.google.com/books?id=YVi8HVN-APwC&lpg=PA27&dq=computer%20buss%20-sam&pg=PA27#v=onepage&q=computer%20buss%20-sam&f=false |archive-date=January 17, 2018}}</ref> It encompasses both [[Computer hardware|hardware]] (e.g., wires, [[optical fiber]]) and [[software]], including [[communication protocol]]s.<ref>{{cite web |url=https://www.pcmag.com/encyclopedia/term/39054/bus |title=bus Definition from PC Magazine Encyclopedia |publisher=pcmag.com |date=2014-05-29 |access-date=2014-06-21 |url-status=live |archive-url=https://web.archive.org/web/20150207204630/http://www.pcmag.com/encyclopedia/term/39054/bus |archive-date=2015-02-07}}</ref> At its core, a bus is a shared physical pathway, typically composed of wires, traces on a circuit board, or [[busbar]]s, that allows multiple devices to communicate. To prevent conflicts and ensure orderly data exchange, buses rely on a [[communication protocol]] to manage which device can transmit data at a given time.
Buses are categorized based on their role, such as [[system bus]]es (also known as internal buses, internal data buses, or memory buses) connecting the [[Central processing unit|CPU]] and [[Computer memory|memory]]. [[Expansion bus]]es, also called [[peripheral bus]]es, extend the system to connect additional devices, including [[peripheral]]s. Examples of widely used buses include [[PCI Express]] (PCIe) for high-speed internal connections and [[Universal Serial Bus]] (USB) for connecting external devices.
