Template
A pre-designed layout or framework used as a starting point for consistent production.
In practice
A template is a ready-made layout or framework with fixed elements and placeholders, used as the starting point for many similar pieces. It saves time and keeps output consistent, especially when non-designers produce material. A university might provide a slide template with its logo, colors and title slide fixed, so every department's presentations look related while their content differs.
Often confused with Mockup
A template is a reusable framework for producing new pieces; a mockup is a realistic picture of one finished design, used to present or judge it.
Try this
Build a simple template for event flyers, fill it with two very different events, and note what breaks.