The all fabled "nod of acknowledgement is an interesting phenomenon. It is, put simply a slight head nod exchanged between males. While its use is documented among females, it is a rare and condition specific phenomenon. While exchanged between friends who, for whatever reason may not want to acknowledge each other verbally, in some places of the world, it is a common greeting among strangers. For example, in the southern United States, it is very common to offer a head nod to another male as you pass each other or get caught glancing at each other. However, as you travel further north, the head nod disappears among unfamiliars. While taking a stroll in NYC, you'd be hard pressed to get any unfamiliar male to return a nod of acknowledgement while simply walking down the street.
Gary and his girlfriend step into the crowded living room, the party in full swing. They notice Tim and his girlfriend. Gary and Tim exchange a nod of acknowledgement across the loud and crowded room, and then return to scanning the party. The two girlfriends do something along the lines of squealing or waving or running to each other and immediately compliment each other's shoes.
2. Dale walks down the street. He passes some guy, and they exchange a nod of acknowledgement.
3. As Lars stands in line at the hardware store scoping out the others waiting in line, he inadvertently catches the eye of Sven, two isles over. They simply exchange a nod of acknowledgement and look away.