"Penile erection" refers to the condition when the penis becomes stiff and gets bigger and harder. It happens when a man is sexually aroused or stimulated. Full definition
It concluded that Pfizer knew and did not properly disclose that sildenafil is the only compound in Viagra, out of several listed compounds, that induces penile erection in impotent males. (businessinsider.com)
But Pfizer's patent only states that «one of the compounds (in Viagra) induces penile erection in impotent males, without specifying which is the effective compound.» (businessinsider.com)
The researchers felt this reaction was «an incomplete or abortive form of penile erection.» (discovermagazine.com)