Defining the "left" becomes more challenging when you survey all the populations of the world.
"Left-wing" is quite a bit different in some countries than it is in the USA.
Very strictly speaking, the Left vs. Right dichotomy is an 18th century French conception. You know, those who favored the Catholic Church, the Monarchy and the Aristocracy were the Right, and those in favor of Secularism, Republicanism and Democracy were the Left. All modern use of Left vs. Right dichotomies can all be traced back to the French Revolution and if you use the Left-Right dichotomy in the context of nominally proclaimed values, the definition of Left and Right can be applied fairly consistently and universally across time and culture. The Left favors equality and progress, the Right order and tradition. The 18th century Left promoted Laissez-Faire Capitalism in order bring down the Aristocracy, for the same reasons the Left may now advocate government intervention: To achieve equality.
In certain countries (China comes to mind) the Left-Right dichotomy is rarely heard in common political discourse. Chinese politics are divided between Centralists vs. Decentralists. It's quite reasonable European politics will be divided among the same lines, due to the EU, and the questions and opinions such will arouse. In many ways, this is already the case: In the Netherlands, the SP and PVV, who are considered polar opposites in Left-Right dichotomy are actually quite similiar when viewed in terms of Unificationist vs. Eurosceptic sentiment, with the SP and PVV belonging to the latter. Another (Dutch) example, is the Dutch Republic, where - particularly in the 18th century - its politics were marked by the battle between the
Staatsgezinden and
Prinsgezinden. I won't bore you guys with further Dutch history, but the bottomline is that neither can be solidly classified by classic Left-Right dichotomies. In a way, American politics can't be classified in Left-Right dichotomies either: For example, the Republicans have consistently favored the interests of the Northeastern elite and industrialists since their very inception, with the Democrats consistently opposing them. For a long time, the Democrats would've been considered Right of the GOP in Left-Right dichotomy, due to its support from Southern Farmers, but now it (onstensibly) supports Leftist values. Yet the interests have remained the same, with the Northeastern Elites and Industrialists being represented by the Republicans and those who oppose the interests of the GOP's constituents, whether Southern farmers, Progressives, Minorities, etc. supporting the Democrats.
The Left-Right dichotomy is becoming increasingly irrelevant, if it wasn't already so. Besides, it is for most part a rethorical scheme as well, considering how Left and Right parties are converging policy-wise. But as said earlier, all things termed "left" are associated with rethoric about equality and progress, and all things termed "right" usually like to talk about order and tradition. Therefore, IMO, it is better to understand politics in terms of the interests of political parties' constituencies, and the policies these inevitably lead to, rather than arbritrary dichotomies, however clearly they may be demarcated if an effort is made to do so.