all firms paying the highest salary for first year associates can be considered to be BIGLAW
BigLaw has historically been known as the AM Law 250. I like to think of only the top 100 AM Law Firms, but that would disqualify a lot of national firms that are not as large. So to answer the question, when we speak of BigLaw, we generally mean the AM Law 250 (which includes the V100).
since this thread has been revived, I should point out that there is no AmLaw 250. There is an AmLaw 200 and a NLJ 250. The AmLaw 200 is the top 200 firms by gross revenues, and the NLJ 250 is the top 250 firms by number of attorneys. I think the AmLaw 200 is the more useful measure, personally, for determining what biglaw is. Which one were you talking about?