Zephyr (/ˈzɛf ər/) is a small real-time operating system (RTOS)^[7] for connected, resource-constrained and embedded devices (with an emphasis on microcontrollers) supporting multiple architectures and released under the Apache License 2.0. Zephyr includes a kernel, and all components and libraries, device drivers, protocol stacks, file systems, and firmware updates, needed to develop full application software.^[8]

It is named after Zephyrus, the ancient Greek god of the west wind.^[9]

Zephyr originated from Virtuoso RTOS for digital signal processors (DSPs).^[10][11] In 2001, Wind River Systems acquired Belgian software company Eonic Systems, the developer of Virtuoso. In November 2015, Wind River Systems renamed the operating system to Rocket, made it open-source and royalty-free.^[11] Compared to Wind River's other RTOS, VxWorks, Rocket had a much smaller memory needs, especially suitable for sensors and single-function embedded devices. Rocket could fit into as little as 4 KB of memory, while VxWorks needed 200 KB or more.^[11]

In February 2016, Rocket became a hosted collaborative project of the Linux Foundation under the name Zephyr.^[10][12][1] Wind River Systems contributed the Rocket kernel to Zephyr, but still provided Rocket to its clients, charging them for the cloud services.^[13][11] As a result, Rocket became "essentially the commercial version of Zephyr".^[13]

Since then, early members and supporters of Zephyr include Intel, NXP Semiconductors, Synopsys, Linaro,^[14] Texas Instruments, DeviceTone, Nordic Semiconductor, Oticon, and Bose.^[15]

As of January 2022^[update], Zephyr had the largest number of contributors and commits compared to other RTOSes (including Mbed, RT-Thread, NuttX, and RIOT).^[16]

Zephyr intends to provide all components needed to develop resource-constrained and embedded or microcontroller-based applications. This includes, but is not limited to:^[8]

• A small kernel
• A flexible configuration and build system for compile-time definition of required resources and modules
• A set of protocol stacks (IPv4 and IPv6, Constrained Application Protocol (CoAP), LwM2M, MQTT, 802.15.4, Thread, Bluetooth Low Energy, CAN)
• A virtual file system interface with several flash file systems for non-volatile storage (FatFs, LittleFS, NVS)
• Management and device firmware update mechanisms

• Embedded operating system