What's the definition for the intensity of em radiation ?

Radiation Intensity is measured in Watts per steradian. (Watts per solid angle). The average intensity, then, is ( Power radiated ) / ( 4 pi ). Radiation *Density*, however, incorporates distance and area, using: ( Power radiated ) / ( 4 pi r^2 ), where r is the radius from the antenna, and your answer will be watts per unit area. If r is in metres, then density will be watts per metre^2.

I don't know what a definition might be, but intensity is what we call brightness when the EM radiation is visible light. And brightness is the number of photons per unit time striking the retina. Clearly, the larger the area of the retina, the more photons for a given source there will be over time. We have a similar case with radio/TV reception when using an antenna. Antennae with larger apertures, a measure of reception area, will receive a more intense signal, than smaller antennae. This is why, for example, the so-called radio telescopes are very large... to receive a more intense of an otherwise weak signal from the stars. So there you are. Intensity is proportional to the rate of number of photons over time and over some given area. We can write a shorthand expression for that as I ~ dN/dt X A. When thinking of EM radiation as a wave, dN/dt ~ amplitude squared. That is, the greater the amplitude of the wave, the more intense will be that EM radiation because the number of photons over time is proportional to the square of the amplitudes.

Intensity means "how strong" the field is. It is generally measure as "power per square unit", typically "watts per square meter" or some fractional power such as milliwatts, microwatts, etc. The area can also be expressed in square millimeters, inches, etc.

If you're dealing with basic antenna theory, then I believe intensity is refers to the magnitude of the Poynting vector, or |E|²/(2ŋ) for the plane-wave approximation. It may also refer to |E|.

I don't know what a definition might be, but intensity is what we call brightness when the EM radiation is visible light. And brightness is the number of photons per unit time striking the retina. Clearly, the larger the area of the retina, the more photons for a given source there will be over time. We have a similar case with radio/TV reception when using an antenna. Antennae with larger apertures, a measure of reception area, will receive a more intense signal, than smaller antennae. This is why, for example, the so-called radio telescopes are very large... to receive a more intense of an otherwise weak signal from the stars. So there you are. Intensity is proportional to the rate of number of photons over time and over some given area. We can write a shorthand expression for that as I ~ dN/dt X A. When thinking of EM radiation as a wave, dN/dt ~ amplitude squared. That is, the greater the amplitude of the wave, the more intense will be that EM radiation because the number of photons over time is proportional to the square of the amplitudes.

Intensity means "how strong" the field is. It is generally measure as "power per square unit", typically "watts per square meter" or some fractional power such as milliwatts, microwatts, etc. The area can also be expressed in square millimeters, inches, etc.

If you're dealing with basic antenna theory, then I believe intensity is refers to the magnitude of the Poynting vector, or |E|²/(2ŋ) for the plane-wave approximation. It may also refer to |E|.