SNIA Online Dictionary

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U.2	[Hardware] The SFF-8639 connector used for a Quad (4x) PCIe bus.
UDP	[Network] Acronym for User Datagram Protocol.
UID	[Management], [Data Security] Short for "user identifier" (User IDentifier).
ULP	[Fibre Channel] Acronym for Upper Level Protocol.
Ultra SCSI	[SCSI] A form of SCSI capable of 20 megatransfers per second. Single ended Ultra SCSI supports bus lengths of up to 1.5 meters. Differential Ultra SCSI supports bus lengths of up to 25 meters. Ultra SCSI specifications define both narrow (8 data bits) and wide (16 data bits) buses. A narrow Ultra SCSI interconnect transfers data at a maximum of 20 MBytes per second. A wide Ultra SCSI interconnect transfers data at a maximum of 40 MBytes per second.
Ultra2 SCSI	[SCSI] A form of SCSI capable of 40 megatransfers per second. There is no single ended Ultra2 SCSI specification. Low voltage differential (LVD) Ultra2 SCSI supports bus lengths of up to 12 meters. High voltage differential Ultra2 SCSI supports bus lengths of up to 25 meters. Ultra2 SCSI specifications define both narrow (8 data bits) and wide (16 data bits) buses. A narrow Ultra SCSI interconnect transfers data at a maximum of 40 MBytes per second. A wide Ultra2 SCSI interconnect transfers data at a maximum of 80 MBytes per second.
Ultra3 SCSI	[SCSI] A form of SCSI capable of 80 megatransfers per second. There is no single ended Ultra3 SCSI specification. Low voltage differential (LVD) Ultra2 SCSI supports bus lengths of up to 12 meters. There is no high voltage differential Ultra3 SCSI specification. Ultra3 SCSI specifications only define wide (16 data bits) buses. A wide Ultra3 SCSI interconnect transfers data at a maximum of 160 MBytes per second.
Ultrium	[Storage System] The half-inch, 'square' tape implementation of the LTO format, currently in its 4th generation, LTO-4 Ultrium.
UML	[Management] Acronym for Unified Modeling Language.
unauthorized disclosure	[Data Security] The exposure of information to individuals not authorized to receive or access it.
unicast	[Network] A single transmission of a message to a single receiver connected to a transport network. Unicast is contrasted with broadcast (sending a message to all receivers on a network) and multicast (sending a message to a select subset of receivers).
Unicode	[General] A set of standards intended to allow representation of every character in each of the world's languages; usually understood to mean the 16-bit variant. Sixteen-bit Unicode allows for up to 216, or 65,536 characters, each of which may have a unique representation. It accommodates numerous non-English character sets and symbols, and is therefore an aid to development of products with multilingual user interfaces. Sixteen bits are not enough, however, to represent all the several hundred thousand Asian ideograms. Other 32-bit variants are available for these, but the increased inefficiency in representation of Western text inherent in them has been a barrier to widespread adoption.
unidirectional authentication	[Data Security] Authentication that provides one party to a communication with assurance of another's identity.
Unified Modeling Language	[Management] A visual approach that uses a variety of diagrams such as use case, class, interaction, state, activity and others to specify the objects of a model and their relationships. Various tools exist for turning UML diagrams into program code.
Uniform Resource Identifier	URI Compact sequence of characters that identifies an abstract or physical resource. See RFC2396.
Uniform Resource Locator	URL Compact sequence of characters that identifies an abstract or physical resource, including its location. See RFC1738.
Uninterruptible Power Source	[General] A source of electrical power that is not affected by outages in a building's external power source. UPSs may generate their own power using generators, or they may consist of large banks of batteries. UPSs are typically installed to prevent service outages due to external power grid failure in computer applications deemed by their owners to be "mission critical."
unmap	[Storage System], [Operating System] See trim. Removal of the virtual addresses from a portion of a file (e.g., POSIX).
unstructured data	[Data Management] Data that cannot be easily described as structured data. In general any non-database filesystem content is considered to be unstructured.
Upper Level Protocol	[Fibre Channel] A protocol used on a Fibre Channel network at or above the FC-4 level. SCSI, SBCCS, and NVMe are examples of Upper Level Protocols.
UPS	[General] Acronym for Uninterruptible Power Source.
usable capacity	[Storage System] Synonym for formatted capacity.
user data extent	[Storage System] The protected space in one or more contiguously located redundancy group stripes in a single redundancy group. In RAID arrays, collections of user data extents comprise the virtual disks or volume sets presented to the operating environment.
user data extent stripe depth	[Storage System] The number of consecutive blocks of protected space in a single user data extent that are mapped to consecutive virtual disk block addresses. In principle, each user data extent that is part of a virtual disk may have a different user data extent stripe depth. User data extent stripe depth may differ from the redundancy group stripe depth of the protected space extent in which it resides.
User Datagram Protocol	[Network] An Internet protocol that provides connectionless datagram delivery service to applications. UDP over IP adds the ability to address multiple endpoints within a single network node.
User Identifier	[Management], [Data Security] A unique number that identifies an individual to a computer system. UIDs are the result of authentication processes that use account names, passwords and possibly other data to verify that a user is actually who she represents herself to be. UIDs are input to authorization processes that grant or deny access to resources based on the identification of the requesting user.
userid	[Management], [Data Security] Shorthand for User Identifier.
UTF-8	[General] An encoding for multi-byte character schemes such as Unicode, in which ASCII text encodes to itself and POSIX string manipulation routines work largely as expected. A 16-bit Unicode string that encodes ASCII text will have every other data byte within it set to zero. The UTF-8 encoding of this text will have a zero only at the end.
utilized capacity power efficiency	[Storage System] The ratio of bytes stored on an idle storage system to the amount of power required to maintain the system in a ready idle state.
UUID	[Computer System] An identifier that is expected to be universally unique across systems, space and time.