Know somebody by sight
Definition 1 of 1
Pro Tip 1/3Visual Recognition
This phrase implies you can visually recognize someone but lack personal connection or interaction. 
At company events, I know many colleagues by sight but haven't worked with them directly.
Pro Tip 2/3Acquaintance Level
Use this phrase to describe a low level of acquaintance, mainly through frequent visual encounters. 
I know some parents by sight from school meetings.
Pro Tip 3/3Not for Introductions
Avoid using this phrase when introducing someone, as it indicates you're not personally acquainted. 
I can't introduce you to her; I only know her by sight.