A hub is a node that has a higher degree than other nodes in the graph. How this translates in practice seems to vary between papers. Some just use an arbitrary threshold either as a fixed number or as a percentage, others choose a threshold based on some statistical argument e.g. outliers in the distribution when assuming a scale-free graph.
As a side note, be careful about what you say about high-degree genes/proteins. Many of them have high degrees simply because they have been more studied (e.g. the historical oncogenes such as the RAS family, SRC, EGFR...) or because they belong to a particular class (e.g. ubiquitin-like families, chaperones) or because they are sticky proteins (some are well documented false-positives such as ferritin and ribosomal proteins for yeast two-hybrid or collagens in AP/MS, see also for example the crapome database for "contaminants" in AP/MS).
Finally, note that the scale-free assumption probably doesn't hold for many biological networks.