Question

what is meant by intensity?

Answer 1

Hello !!...

Intensity Means high or extreme degree, as of cold or heat.

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Answer 2

нєуα gυуѕ
Intensity (for any radiation) is the number of photons (packets of energy) falling on a certain area, within some time interval.
Intensity is more if the time interval (t) is short, or lesser the area (a),
proportional to number (n) of photons (I use the symbol >0) & is inversely proportionalto 't'& or 'a'. 
Intensity (I) >0 n ,
I >0 1/t ,
I >0 1/a .
Combining all 
I >0 n/ta.
Now each of these photons (assuming them monochromatic, of single wavelength and frequency, v)
has Energy, E = hv with 'h' as Planck's constant
I >0 n(hv)/ta.
Since I, a, t are in different units and dimensions, we need a balancing 'constant' (k) to make an equation
I = k [nhv/ta].
Fortunately for us, all are in compatible units making k=1 (just ignore 'k')
I = nhv/ta.
By combining 'nhv' as Energy E
I = E / ta in Joules per metre^2, per second.
We know it already that 
Joules per second is expressed in Watts (Power). So
I = P / a in Watts per metre^2.