A starter watch in my opinion is one you buy for more than just looks and for more than just an accessory. Some may spend $100 or others may spend $5000. My idea of a "starter" watch is something that you buy as a beginning to a "collection."
If you would like a better term then you're welcome to offer one yourself. I've heard the term used in many different circles of people so I'm not the only one using it.
A starter watch is a watch you buy at the start of "your" collection. Depending on what you want to spend, it can cost $25 or $25K or more.