In the context of most landscaping projects, yes. A retaining wall is any structure that holds back soil on one side. So even a simple raised planter counts as a retaining wall.
But, you can’t just go into this project without the proper planning, and some support walls do require outside help.
The more soil your retaining wall must support, the stronger both the materials and construction must be. You also need to account for sloping ground, which will increase the amount of pressure on your wall. If you’re not 100% confident in your engineering and building skills, you might want to leave this project to a professional.
Some areas do require permits to build retaining walls over a certain height. It’s best to check your municipality’s guidelines ahead of time, just to be safe.