This seems pretty self explanatory to me:

Programmable automation controller

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A programmable automation controller (PAC) is a compact controller that combines the features and capabilities of a PC-based control system with that of a typical programmable logic controller (PLC). PACs are most often used in industrial settings for process control, data acquisition, remote equipment monitoring, machine vision, and motion control. Additionally, because they function and communicate over popular network interface protocols like TCP/IP, OLE for process control (OPC) and SMTP, PACs are able to transfer data from the machines they control to other machines and components in a networked control system or to application software and databases.

Don't believe Wikipedia on this, its strictly advertising hype and it depends on the manufacturer. Take for example Rockwell's line of controllers; they group their PLC-5, SLC, Micrologix, and PICO controllers under the PLC description. The Logix family are said to be in their PAC line of controllers. Other companies, like National Instruments, labels their PC based controllers as PACs. I've used both and there is really no direct comparison between Rockwell's PACS and National Instruments; they are both effective tools but for different types of work..

Simple answer:

PAC is a term coined by ARC (Automation Research Corporation) to define a greater functionality than PLC.

In particular used for multiple functions across multiple applications - networking etc, check out:

http://www.opto22.com/site/le_demos_demo8.aspx

http://www.controlviews.com/articles/PAC.html

Machine tool or process Industry earlier depended on electrical relays for automatic control .With the development of computers , Microprocessors of took over with pre set programme. Then came automation with PLC (Programmable Logic Controller) A programmable microprocessor-based device that is used in discrete manufacturing to control assembly lines and machinery on the shop floor .PLC was used for many other types of mechanical, electrical and electronic equipment in a plant. This used RISC based and programmed in an IEC 61131 programming language. Sensors and actuators were connected through number of I/O ports to provide automatic control systems. Various manufacturers started producing chips manufactured and programmed for specific purpose which they termed as PAC .The function is almost similar but they customised them to specific application use. In short purpose is same but different manufacturers trade marked the individual PAC using specific chips and programmes for specific needs.