Dominance to me is how much a player stands out among his peers. Things like winning the scoring race by 69% (Gretzky in 1987) or winning the goal scoring title by 69% (Brett Hull 1991) are dominant. These players stood out among their peers.
Dominance can also be based on eyewitness accounts, not just scoring totals. For example, a player like Orr won 8 Norris trophies (voted on by hockey writers who watched the games of the era), that is dominance. Finishing consistently in the top 3 or 5 voting for the important awards (Norris, Hart, Vezina) over the span of 5, 10 or 15 years is dominance as well. Winning multiple Conn Smythe trophies resonates with me as well. That is standing above your peers, IMO.
**Since the Vezina has only been voted on since 1982, the 1st all star goalie position is the "real" Vezina for years previous.**
To me, being dominant does not necessarily mean you are the best player in your era but, that you are among the best 3 - 5 players of your era consistently for several years.
Dominant players are players like: Gretzky, Howe, Orr, Lemieux, Shore, Beliveau, Hall, Roy, Bourque, Esposito, Jagr, Richard, Harvey, Sawchuk, Dryden, Hasek, Mikita, Kennedy, Clancy, Coffey, Kelly, Cowley, Morenz, Lafleur, Bossy, Trottier.
Players that I do not consider dominant would include: Gartner, Francis, Sundin, Ciccarelli, Anderson, Sittler, Nicholls, Foote, Gillies, Federko, Weight, Modano, Larmer. They weren't among the top 5 in the league for an extended period of time. Top 20 or 25? Sure. But, that lands them outside my definition of dominant.