A proximity-actuated field was a personal force field which impeded positron flows and kept potential assailants at bay.
In 2366, merchant and collector Kivas Fajo carried a device which generated this field to protect himself from Data, whom he kidnapped and faked the death of. This field prevented Data from grabbing Fajo, and a second attempt resulted in Data being thrown away from him. According to Fajo, exposure was very bad for the brainpaths in the long term. Fajo used the field again after Data ignored his commands to behave when he attempted to show off the sentient android to his friend Palor Toff. (TNG: "The Most Toys")