A crush on someone that developed because of admiration for the person's skill/talent/intellectual abilities etc. Often used by straight(er) people to clarify that crushes of the same gender are more or less platonic.

When you have a crush on somebody whom you would not normally go for. You become nervous while talking to them, or sometimes even just talking about them. The crush is a type of crush for all ages but when you have a nerd crush it is like you are in middle school again.
Not meaning you have a crush on a nerd but that YOU are a nerd because you get nervous when you talk to them, start sweating profusely, etc.

a nerd crush is a crush you have on a certain nerd. if you had to choose between a sea of nerds, you'd choose this nerd in a heartbeat. you do not necessarily desire to date this nerd, but he could potentially be a candidate.