What exactly is the neural receptive field?
I think is closer to 2. It is the subset of sensory space that elicits a reaction in a given neural population.

So you should:


Pick your neural population. Usually it is a single neuron (but can be easily extrapolated to bigger populations). It may be a sensory neuron, or a neuron in a higher processing region.
Determine your sensory space. That is, you should decide the kind of stimuli you'll be dealing with. For example, not only "visual stimuli", but you should identify the parameters, or independent variables, you will vary in your experiment.
Define a criterion to discriminate between reaction and non-reaction. This implies to choose what is the dependent variable in your study (for example neuron firing rate). Then you could define what features in the relation $\frac{dependent\:variables}{independent\:variables}$ you will search for. That is the tuning curve (which well may be a high dimensional curve).
Perform your experiment and analyze your curve. The region of parameters (independent variables) whose reaction goes above threshold according to your criterion is the receptive field. Sometimes is not trivial to define such a criterion (see: http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.0040092)


A good general reference is the article in Scholarpedia: http://www.scholarpedia.org/article/Receptive_field

This type of analysis is very useful for position coding, that is, when you expect that the activation of a specific region in the brain correlates to stimuli in a region of sensory space. But sometimes things are more complicated (e.g Olfactory coding http://www.jneurosci.org/content/30/27/9017.full).