Can we talk about a portrait photograph if, let's say, a grandmother and her 3-year-old grandchild are in the picture? Or, must it be only one person in the photograph?
One can find other more or less "official" definitions of portrait photography, but this one captures several aspects that are important to portrait photography (or to portraiture in the visual arts overall) which may not be explained in detail in a general-purpose dictionary.
By the very existence of the term "group portrait", clearly such a thing exists. One can also say "individual portrait", but generally the implication of the term alone is that a single person is portrayed. However, if there are multiple subjects — the grandmother and granddaughter, for example — the picture isn't automatically a portrait without some of the above.
A successful photograph of a grandmother and granddaughter might be thought of as two portraits in one: first, a portrait of the grandmother, showing her personality through her relationship to the child; second and simultaneously, a portrait of the granddaughter, showing her personality through her relationship with the older woman.
A lesser photograph might succeed at just one of these, being effectively a portrait of one person with the other person as a prop. Or, if the focus is on the activity of the two subjects, or on their surroundings, it's probably not really a portrait.
I'll go slightly beyond the simple "a picture of one or more people", because there are differences between portraits and pictures that include people.
There are lenses intended as 'portrait lenses' - but the focal length or perspective or f-stop doesn't make an image a portrait.
I'll argue that if the 'focus' of an image is a person (or group) then it is a portrait. Hopefully it will in some way inform you the subject(s).
A portrait may attempt to highlight a relationship between people, or with an environment or an object.
This is opposed to another image which simply happens to contain people.
Voting down seems petty so I didn't (not even sure if I can), but this is a simple dictionary question. A portrait is a representation of a person or group of people.
EDIT: clarification to remove unintended snark: A representation of an individual or of a group (say a sports team) are recognisably portraits whereas a street photograph which happens to include people is not. If there is a distinction it is in intent, if the principal subject is one or more people it is probably a portrait, otherwise not. Whether photography, pencil sketches or paintings.