A particularly common English idiom, "love at first sight" is a strong and immediate attraction to someone upon meeting, and seeing, that person for the first time.
The implication is that this attraction is due to external factors: physical attractiveness, sex appeal, and so forth. It is not based on knowledge of the other person's thoughts or personality.
Example: "When Billy met Cassie, it was love at first sight. He was stunned by her casual charm and elegant figure."