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Ordinal number
Ordinal numbers (or ordinals) are numbers that show something's order, for example: 1st, 2nd, 3rd, 4th, 5th.
Suppose a person has four different Tshirts, and then lays them in front of the person, from left to right.
 At the far left, there is the red Tshirt.
 Right of that is the blue one.
 Then there is the yellow one.
 And finally, at the far right is an orange Tshirt.
If the person then starts counting the shirts from the left, he would first see the red shirt. So the red shirt is the first Tshirt. The blue shirt is the second Tshirt. The yellow shirt is the third one, and the orange Tshirt is the fourth one.
The first, second, third, and fourth in this case are ordinal numbers. They result from the fact that the person has many objects, and they give them an order (hence 'ordinal'). The person then simply counts those objects, and gives the ordinal numbers to them.
In set theory, ordinals are also ordinal numbers people use to order infinite sets. An example is the set [math]\omega_0[/math] (or [math]\omega[/math] for short), which is the set containing all natural numbers (including 0).^{[1]}^{[2]} This is the smallest ordinal number that is infinite, and there are many more (such as [math]\omega[/math] + 1).^{[3]}
Related pages
References
 ↑ "Comprehensive List of Set Theory Symbols" (in enUS). 20200411. https://mathvault.ca/hub/highermath/mathsymbols/settheorysymbols/.
 ↑ "Cardinal and ordinal numbers". https://www.win.tue.nl/~aeb/at/ordinals.html.
 ↑ Weisstein, Eric W.. "Ordinal Number" (in en). https://mathworld.wolfram.com/OrdinalNumber.html#:~:text=In%20formal%20set%20theory,%20an,52;%20Suppes%201972,%20p..

