A party can considered to be social democrat when it advocates in opposition and implements in office using the levers of state power to alter the processes and outcomes of the capitalist market system, most obviously by providing an extensive social wage and through public ownership. The height of social democracy was in the post-war period, from 1945 to the mid-1970s and is often known as the ‘post-war settlement’ and most keenly identified with the welfare state. Hence, Wikipedia says:
Social democracy is a political, social and economic ideology that supports economic and social interventions to promote social justice within the framework of a liberal democratic polity and capitalist economy. The protocols and norms used to accomplish this involve a commitment to representative and participatory democracy; measures for income redistribution and regulation of the economy in the general interest; and welfare state provisions. Social democracy, thus, aims to create the conditions for capitalism to lead to greater democratic, egalitarian and solidaristic outcomes, and is often associated with the set of socioeconomic policies that became prominent in Northern and Western Europe—particularly the Nordic model in the Nordic countries—during the latter half of the 20th century.