The radio frequency band historically used for a type of wireless network that blends walkie-talkie (push-to-talk) functionality with a cellular phone network. The most prominent implementation in the US was iDEN technology used by Nextel and branded Direct Connect.
Nextel was bought by Sprint, and Sprint now uses the ESMR band for CDMA service.
The ESMR band is near 800 MHz. The precise frequencies include 806 - 825 MHz and 851 - 869 MHz.
Nextel uses other bands closer to 900 MHz as well, the largest of which is called the ISM band. These have a range of roughly 896 - 927 MHz.
The ESMR band is also known as BC10, which stands for band class 10. BC10 was used in the context of 3G CDMA service.
For 4G LTE service, it's part of band 26.
See: Band 26
It is important not to confuse the ESMR "800" band with the Cellular "800" band, as they are different radio bands with a different, non-overlapping range of specific radio frequencies.