Dose rate in 10 mm tissue depth (directional equivalent dose rate) from a radioactive source.
Ambient Dose Equivalent H*(10) is a quantity now widely accepted. In case of photons, it is similar to Hx except that it accounts for absorption and scattering of radiation by the human body. The definition of H*(10) simulates the human body through a phantom (the ICRU sphere, a sphere of 300 mm in diameter made of tissue equivalent material). H*(10) is the dose equivalent at a depth of 10 mm inside that sphere, that is at the place of an assumed inner organ. An instrument measuring dose (rate) free in air must simulate by design the effect of the ICRU sphere if it shall measure H*(10). Then this instrument will provide a good estimate for the dose a person would receive if he/she finally went to that place.
Source: Radiation Quantities and Units