In my opinion first impression is when you meet someone for the first time or you talk to them for very first time. Most of us do form our opinion in very few seconds but i have learned that most of the times we are wrong of our opinions based on first impression. So people with experience never limit the judgement on first impression. Though i would recommend that having a good first impression is very important because it is a positive sign for you and people are interested in knowing you and in case of bad first impression people tend to have a negative opinion of you and are not much interested in knowing you.

Actually only 7 seconds is the average length of time you have to make a first impression. If your first impression is not good , you won' t get another chance with that person , a potential client or for a job.

I believe so, why it is called "first impression", we always judge a person by our first sight.