A hub is surely the central component of a hub and spoke network where a large proportion of traffic takes advantages of waves of connections. A large % of passengers will transit hub C on the way from A to B.
DL : ATL, JFK, DTW, MSP (SEA?)
UA : ORD, IAD, EWR, IAH
AA : DFW, ORD, MIA, JFK
US : PHL, PHX, CLT
<ready to edit as I'm sure I've missed one>
Delta also market SLC, CVG and MEM as their hubs. Although interestingly they only have one trans-Atlantic route (to CDG) from SLC and CVG.