1. The power or magnitude of light, heat, or whatever else you might analyze in physics class. Intensity is inversely proportional to the square of distance from the source.
2 (a) Excessive and mindless enthusiasm for an activity which is often trivial or pointless. This concept was coined by a group of students from
radnor high school when they were bored during gym class. When you’re playing soccer, Intensity is the voice in your head that tells you to tackle the goalie. Intensity is the Mortal Kombat theme song playing on the nearest boom box when everyone in your gym class has a hockey stick. At the same time, Intensity is getting pumped over something irrelevant. You would be a normal fan if you got insanely pumped for a footbal game, but you would be Intense if you got insanely pumped for a football game between a mediocre team and a really bad team (see Lower Merion High School and radnor). Other legit examples are shouting "defense" as other physics olympics teams try to get their rubber-band powered airplanes off the ground, getting in a fight with another school's mascot, and spirited douche-baggery.
Intensity is generally accepted to be equal to perspiration multiplied by duration. With this meaning, NEVER use the original pronunciation. A good bet is In-tense-it-eye.
2 (b) Getting rowdy

A burning, incredible sensation. or a hectic moment.

To be all tensed up and rigid, like a coiled cobra that's ready to strike at any moment.

a word that can be used at the end of almost any sentence. often used to describe an event in which something very big has happened and the only word you reach for is intense. you can always use this word while speechless and someone is looking for a response.

a word used to describe the intensity of a moment.
Usually meaning sick, crazy or plain AMAZING
Can also be used as giving head on a plane.

To be amazingly awesome without trying.

Used to describe people that try too hard to be "cool", people that try to get attention by doing something "cool", or people that show off or act cocky in a miserable attempt to impress others.