A receptive field is an area in which stimulation leads to response of a particular sensory neuron.
A receptive field is often functionally characterized by electrophysiological experiments. During these experiments, a certain area of the body is stimulated: e.g., a certain part of the visual field is illuminated, or a certain area of the skin is haptically stimulated (Fig. 1), etc.. Then it is determined if a neuron shows a response, either a stimulation (increased firing rate), or inhibition (decreased firing rate). By probing multiple, overlapping areas, the RF can be characterized.
Levine and Shefner (1991) define a receptive field as an "area in which stimulation leads to response of a particular sensory neuron".
The RF is hence a property of the neuron and not of the stimulus.
However, that property of the neuron depends heavily on the stimulus. For example, an ON retinal ganglion cell will not respond when the, say, red-cones in the center of its receptive field in the retina are illuminated with UV light, while it will vigorously respond when the light is red. It is here where your professor may have been hinting at, i.e., at the close interplay between stimulus and response.
Fig. 1. Schematic of the receptive field (RF) of a pain in the skin. The RF is determined by the arborations of the free nerve endings in the dendritic zone of the pain receptor cell. source: Lumen Learning