This term is often used to refer to a PCIe slot based device. With the advent of storage devices using NVMe, it is possible plug an SSD directly in to a PCIe slot. In this case the SSD could be described as an ‘AIC drive’.
Lifewire describes PCIe in more detail.
Enterprise and Datacenter Storage Form Factor
A high density hot-swappable form factor for enterprise storage systems. See also: M.3/NGSFF, and Read our blog on EDSFF
This is a developing specification for the interconnection of storage and processing elements of a computer system. It is designed for low latency and has a lightweight software stack.
AnandTech reports: Gen-Z consortium adds support for PCIe Gen 5
M.2 is most commonly used for solid-state storage in laptops. They are also found in some enterprise storage systems and consumer motherboards. The interface can be used for either SATA or NVMe-based drives and the connector can be ‘keyed’ to prevent the insertion of some drives.
M.3 / NGSFF
Next Generation Form Factor (originally known as M.3)
This is a competing standard to EDSFF, and is also intended for high density storage using small NVMe drives. It extends from M.2, but appears that it will not be compatible, despite using the same edge connector.
AnandTech’s article – PCI-SIG Warns Of Incompatibilities Between M.2 And Samsung’s NGSFF/NF1
Non-Volatile Memory Express
NVMe is a protocol designed for use with solid state disks. It was designed to allow SSDs to be attached to the PCIe bus and make best use of the low latency of flash memory in comparison to spinning drives.
Quad Small Form-factor Pluggable
A quad (4 lane) form of the SFP interface, allowing 4 times the data throughput. This is a larger connector that SFP.
Serial Attached SCSI
SAS is a very common enterprise storage interface. Normal speeds are from 3G (3 Gigabits/s) to 12G, with 24G speeds now emerging. SAS is used both for connection between a disk and the host and as a back-of-rack cabling interface. Multiple SAS lanes are often used to increase the interface capacity.
Serial Advanced Technology Attachment
A very common storage interface found on almost all consumer motherboards. SATA is sometimes used for enterprise purposes (when cost is important) but is generally used in slower, less critical consumer devices. SATA devices can be used in SAS systems, though SATA typically only has a single data port.
Small Form Factor
This is a common term for many technologies. In storage it is generally used to describe the common 2.5 inch drive bay standard.
Small Form-factor Pluggable
The SFP interface is a common, hot-pluggable, network interface. It is commonly used for optical ethernet links, though copper cables can be used. Other protocols can be carried, including Fiber Channel and SONET.
This is the connector used for U.2 and U.3 2.5 inch (Small Form Factor) storage devices. It was designed for use with SAS, SATA and NVMe drives. Read our blog on SFF-83639
Separate Reference Independent Spread
The PCIe device and host use different RefClk sources (Separate Reference), so the clocks may not match each other perfectly, and this must be accounted for. The clocks use spread spectrum clocking, reducing EMI (Electromagnetic Interference).
TrueChip.net explains more about SRIS
Separate Reference No Spread
Similar to SRIS, but the clocks do not employ spread spectrum clocking
This is an interface intended for enterprise-grade solid state storage, and also compatible with hard disks. It is one of the most common ways to attach high-end SSDs.
This is a redesign of the U.2 standard, bringing all of the communications standards onto a common set of TxRx pins (unlike U.2 which used different pins for PCIe and SAS). The aim is to have a single drive slot that supports a range of drive types.
For a full explanation, read our blog on U.3