802.15.4
The low-power radio standard underneath Thread and Zigbee.
IEEE 802.15.4 defines a low-rate (250 kbit/s at 2.4 GHz), low-power radio and MAC layer used by Thread, Zigbee and others.
Examples
802.15.4 at 2.4 GHz uses channels 11 to 26, sends 250 kbit/s and carries frames of at most 127 bytes.
The ESP32-C6 and ESP32-H2 have an 802.15.4 radio; the ESP32-S3 does not, so it can never join a Thread mesh.
See also