Concordia, discordia, intercepts, Tera–Wasserburg diagram কী?

The concordia is the curve of the $(x, y)$ plane on which the two uranium–lead ages are equal. A discordia is a straight line along which discordant analyses of one population of crystals are aligned; its intersections with the concordia are the upper intercept (the older) and the lower intercept (the younger). The Tera–Wasserburg diagram plots $X = {}^{238}\mathrm{U}/{}^{206}\mathrm{Pb}^*$ against $Y = {}^{207}\mathrm{Pb}^*/{}^{206}\mathrm{Pb}^*$.