A ‘black hole’ is a body in space which does not allow any radiation to come out. This property is due to its

A black hole prevents any radiation from escaping because its gravitational field is so intense that the escape velocity at a certain boundary (the event horizon) equals or exceeds the speed of light, so light cannot get out [2]. This extreme gravity arises when a large mass is packed into an extremely small volume, producing an immensely high (formally diverging) density and a correspondingly strong gravitational pull; nothing, not even light, can overcome that pull once inside the horizon [1]. Thus the defining reason radiation cannot escape a black hole is the very high density (and resultant gravity) of the object, not merely its size alone.

Sources

1. [2] https://www.britannica.com/topic/event-horizon-black-hole
2. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 1: The Universe, The Big Bang Theory, Galaxies & Stellar Evolution > Black holes > p. 15