What is the term for the reduction in receiver sensitivity caused by a strong signal near the received frequency?

Receiver desensitization, often called "blocking," is the correct term for a severe drop in receiver sensitivity when a strong off-channel signal is present. This typically happens when another station is transmitting very close to your operating frequency. The strong adjacent signal enters the receiver's front end and overloads the RF amplifier or mixer stages. This overload can drive the components into non-linear saturation or cause the receiver's Automatic Gain Control (AGC) to aggressively reduce the overall gain, making weak signals disappear. While "cross-modulation interference" is another type of overload, it specifically refers to the audio modulation of the strong signal transferring onto the desired signal, rather than just a generalized loss of receiver sensitivity.