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Partial eta-squared is a measure of variance, like r-squared. It tells us what proportion of the variance in the dependent variable is attributable to the factor in question. You can get these measures by choosing the 'Estimates of effect size' option when setting up an ANOVA.
ETA: ETA, Basque separatist organization in Spain that used terrorism in its campaign for an independent Basque state. ETA grew out of the Basque Nationalist Party (Partido Nacionalista Vasco; PNV), which was founded in 1894 and which managed to survive, though illegally, under the fascist regime of Francisco Franco by maintaining its headquarters in exile in Paris and keeping quietly out of sight in Spain.