Two rarish, and to me, indespensable words: haecceity (from Latin hæcceitas 'thisness') and quiddity (from Latin quidditas 'whatness'). A more scholarly look at haecceity. Haecceity was coined by medieval philosopher Duns Scottus whose first name gives us the eponym dunce.
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This opinion posits that that one substance [viz. the universal], under many accidents, will be the whole substance of all individuals, and then it will be both singular and this substance of this thing [x], and in another thing [y] than this thing [x]. It will also follow that the same thing will simultaneously possess many quantitative dimensions of the same kind; and it will do this naturally, since numerically one and the same substance is under these [viz. x's] dimensions and other [viz. y's] dimensions. (Scotus, Reportatio Parisiensis II, d. 12, q. 5, n. 3 [Scotus (1639), 11:326b]; see also Scotus, Ordinatio II, d. 3, p. 1, q. 1, nn. 37, 41 [Scotus (1950-), 7:406-7, 409-10; Spade (1994), 65-67])
