Austin Bramwell suggests a definition:
Conservatism is the defense of legitimacy wherever it happens to exist. “Legitimacy” here is defined in the empirical, Weberian sense: that is, an institution is legitimate if and only if the opinion has become widespread that it is right (for whatever reason or lack thereof) to obey it. The conservative, in short, cultivates obedience to existing institutions. This definition, I submit, has all the advantages of the conventional definitions, none of their defects, and some important advantages of its own.
One quibble: why obedience? Why not acceptance or defense of coherent institutions? Razib responds, and tries to define "Liberalism as the inverse of Bramwell's definition of Conservatism."