I guess there is more than one answer to this. The traditional sense would be as fashion models and commercial models are seen and used today, as in the past. They are represented by agencies and the agencies provide models to commercial clients who have a casting for models, to wear and showcase their product, clothing article. It becomes a professional modeling gig when there is a client, and the model is hired to be on set. In that context, the image is captured in such a way to draw attention to the product more so than the subject. And the models are paid for usage and their likeness being used. Commercial models are often more animated, and their mood helps represent the product, or service a company offers. That’s where expression matters. In either case, there are height and measurement guidelines. These professional agency models need to keep updated portfolios, by going to a professional model / commercial / fashion photographer and getting modeling or commercial headshots made, and sometimes full or 3/4 length body shots, depending on their local market and agency requirements.
A pretty girl posing in the mirror making a duckface, or having cute photos of her taken by a friend with a camera, for cool photos to put on Facebook, is not the definition of a model. Regardless of anyone’s opinion of Swimsuit calendars or Playboy type magazines, those are glamour models and they are indeed models as much as any other model, just different style. They are hired by the client, and it’s their ability to draw in the viewer, instead of marketing a product, that sells their image. So in this case, it’s about the subject. There are freelance markets like Model Mayhem, etc. and there are some on there who are professional traveling models and big city agency models….others are hobbyists and just enjoy being photographed, which is great if that’s what they enjoy! Or if their look or stats don’t meet agency requirements, this is their option. By technical definition, they are photo subjects, and those who are shooting for any client for commercial use, are models. This model was shooting with me for a new clothing brand, so there is a client and a commercial usage here. I encourage research and learning how things work before thinking, oh I wanna be a model. Due diligence with all things, and that saves wasted time and disappointment, and chasing after something that isn’t understood.