A first impression is the overall perception of someone that one gleans from a first meeting with that person. Because first impressions are powerful, people are often careful of their demeanor when they meet new people.
One example of the power of first impressions is a study noted by Psychology Today in 2004. In the study, untrained subjects watched 20- to 32-second videos of job applicants greeting interviewers. They were then asked to rate the job applicants on attributes such as assurance and likeability. These untrained subjects gave assessments that were close to those of the trained interviewers, who were given 20 minutes with the applicants. This implies that the very first moments of meeting someone are the most crucial in forming an impression of that person.