No, seriously - there is no straight cut-and-dry answer for this question.
Constantly transferring multiple gigabytes every minute? That's high-traffic.
Constantly handling several million requests per second? That's high-traffic.
A perfume retailer's site that gets one request a day... except the week before Mother's Day and St. Valentine's Day? That's high-traffic, even if the average over the year is decidedly low-traffic.
The best definition I can give you is that "High Traffic" is the point at which:
- You start to worry about bandwidth limitations
- You start to worry about your infrastructure (hardware/software/network) keeping up with the load.
- You know that an outage will cause enough noise that it will be a problem (customers notice and complain)
(this one is also the beginning of the definition for "Business Critical"…)