The definition of ticket severity comes in many flavors depending on opinion. I've seen support sites that have so many to choose from it's overwhelming. Or others that are so vague no one really knows what they mean. This is not only confusing to the customer, but also confusing to the support team.
The natural tendency is to overcomplicate severity. Urgent, High, Medium, Low are the typical ones I see on the net - and ones that I've used before. I've also seen them defined as Level 1, Level 2, etc. Unfortunately this is not helpful to customers, who have no idea what your support levels cover. Likewise, different people define "urgency" differently, so what is "urgent" to one person may not be to another.
Your job, as a customer support provider, is to make your customers' lives easier. This means using simple, easy to understand definitions of ticket severity. Positive examples of software I have used in the past actually provided definitions for each of their severity levels, things such as "business can not operate" or "only one user affected". This is much better, as the customer can choose the most appropriate level of severity, and the rep who gets the ticket knows immediately what the business impact of the issue is, so they can address it appropriately.
Using defined levels also allows your support desk to prioritize issues - those with a higher level of severity such as "business cannot operate" can be assigned immediate priority while lower levels of severity such as "I just have a question" can be addressed later (although not too late - remember all customers are important and responding quickly can be the key to customer satisfaction).
On the note of timely responses, issues with top priority should be answered immediately - even if it is as simple as "we've received your support request, we understand the severity of this issue and have our team working on it right now." For the lower severity requests providing the answer with a few hours is likely to be acceptable.
To keep it simple, think of it this way: Severity = Priority. If you establish severity levels with this in mind, it will help both your customers and your support staff.
Remember, these are just general guidelines - each business has unique needs and you need to set priorities with your own customers in mind.
How do you define severity? Let us know in the comments!