One server per application
In the not too distant past, it was normal to have a one to one relationship between applications and servers. For example, your mail and web applications would reside on two separate, physical servers.
This approach was greatly inefficient and costly too. Each server would be purchased with years of predicted growth in mind. This resulted in racks full of overly powerful and underutilised servers which would drive up cooling, power and data centre space costs.
Furthermore, applications would often fall short of their forecasted growth, leaving businesses in a stalemate. They would want to move their applications to smaller servers to save on costs, but they can’t guarantee they won’t break their applications in the process. If they do break, then that’ll end up costing the business too. It really was a lose-lose situation.