Stereotyping
Stereotyping is expecting a person to have certain traits because they belong to a group, without knowing anything else about them. Stereotypes can be positive or negative, and they often stick even when a person clearly doesn't fit.
In everyday life
A shop clerk assumes an older customer won't understand a new phone and explains every step slowly, though the customer works in tech.
At a party you meet a very tall man and immediately ask which basketball team he plays for, knowing nothing else about him.
Why your mind does this
We rarely have full information about a person or situation, so the brain fills gaps with what usually holds true for similar cases. Relying on categories, rules of thumb and trusted sources saves a huge amount of time and effort. For most of history, following the group and trusting experienced elders was a safe bet.
Its family, in 30 seconds: Stereotypes
How to spot it
Notice when you predict someone's skills, tastes or habits from their age, gender, job or background alone.
How to counter it
Ask the person a genuine question about themselves before acting on any assumption.