A habitable zone (HZ) is a region of space where a planet can hold liquid water (i.e., surface water), which is considered a prerequisite for life as we know it.
In particular, a circumstellar habitable zone (CHZ) is the zone around a star with this potential. The concept was introduced in 1953 and to some extent is now less popular than in the past. The terms liquid water zone and liquid water habitable zone are also used, but other criteria may be of use such as for the size/frequency of stellar flares, etc. The outer edge is basically the system's snow line.
Regarding the solar system, studies vary in their determination of boundaries, but the region from Venus to Mars is considered a rough approximation.
A galactic habitable zone is a region of a galaxy suited to life as we know it, i.e., sufficient metallicity, and an advantageous number of catastrophic events, i.e., supernovae, gamma-ray bursts, etc. Some history of supernovae may be an advantage by contributing to metallicity and star formation.
The amount of received radiation (i.e., the products of radioactive: alpha, beta, and gamma "rays") has been considered regarding habitable zones: a little to assist evolution through mutation but not enough to prevent development. To me, this seems oriented toward the sort of life found on Earth.
An interesting development of the habitable zone concept is the fact that within the solar system, energy from tidal heating appears to be maintaining the temperature for liquid water for more worlds than are at such a temperature due to their distance from the Sun. That suggests that the "likely place for life to exist" could well be moons of giant planets not necessarily in the above-defined habitable zone.