The term was developed to be used to talk about the storage devices that IBM developed, mainly for use in mainframe computers and even some micro computers.
These storage devices have evolved in to the optical drives and hard disks that are found in modern computers today.
Direct-access storage devices allow the host computer to access data directly from wherever it is stored within the storage device because each data chunk is saved in a discrete and separate location from other chunks, complete with a unique address.
One thing to note here, is that no matter where the data is, on the storage device, the speed of retrieval of the data, depends purely on the speed of the storage device.
Example of usage – “Modern DASDs are internal and external hard disk drives that connect directly to the host computer via an IDE, SATA, eSATA, USB or FireWire interface.”