(1) For programming development software, see 
integrated development environment.
(2) (
Integrated 
Drive 
Electronics/
Intelligent 
Drive 
Electronics)  A hardware interface for hard drives and CD/DVD drives.  Introduced in 1986 with 20 megabytes of storage, IDE became the standard, and storage grew to hundreds of gigabytes.  Officially known as an AT Attachment (ATA) device (see 
AT bus), IDE's primary contribution was building the disk controller into the drive itself so that only a simple circuit was required on the motherboard.  Optical drives use the ATA Packet Interface (see 
ATAPI).  Also known as "Parallel ATA" (PATA), IDE/PATA was superseded by Serial ATA (see 
SATA).
Master and Slave
Two IDE drives share one cable and are configured as master and slave.  Out of the box, drives are set to master, and switching jumper pins makes them slaves.
Myriad Names Over the Years
The following mean the same thing: IDE, ATA, IDE/ATA, EIDE, EIDE/ATA, Fast/ATA and PATA.  Using a 100 MB/sec example, drives went by "ATA-100," "Ultra ATA-100," "DMA-100" or "Ultra DMA-100."  For detailed specifications, see 
PATA/SATA specifications.  See 
Cable Select, 
SMART HDD, 
hard disk interfaces and 
SATA.
PATA vs. SATA Cables
IDE/PATA cables took up a lot of room inside the case and impeded air flow.  Motherboards had 40-pin sockets for optical drives and 80 pins for hard drives.
Fault Tolerant IDE
In the late 1990s, Arco was first to provide RAID 1 (mirroring) with "inexpensive" IDE drives rather than SCSI.  This drive bay card plugged into the motherboard, and cables went to two drives (see 
RAID).  
(Image courtesy of Arco Computer Products, Inc.)