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In physics, effusion is the process in which individual molecules flow through a hole without collisions between molecules. This occurs if the diameter of the hole is considerably smaller than the mean free path of the molecules.[1] According to Graham's law, the rate at which gases effuse (i.e., how many molecules pass through the hole per second) is dependent on their molecular weight; gases with a lower molecular weight effuse more quickly than gases with a higher molecular weight. For two gases at the same temperature (and having the same specific heat), and thus having the same kinetic energy, the root mean square molecular speed, vrms, of each gas can be found using the equation
3⁄2kBT = 1⁄2mv2
rms
where kB is the Boltzmann constant. Thus, lighter molecules have a higher speed. This results in more molecules passing through the hole per unit time. This is why a balloon filled with low molecular weight hydrogen deflates faster than an equivalent balloon full of higher molecular weight oxygen.

Source:  wikipedia

In medical terms an effusion is the build up of fluid where it does not belong...+/or can cause harm.

Thus a pleural(lungs) effusion is the build up of fluid in and around the lungs.