The word "thirteenth" refers to the number 13 when counting in a sequence. It is the ordinal form of the word "thirteen" and represents the position of something in a series of thirteen items. Full definition
According to Benoît Godin in Innovation Contested, the term «novation» first appeared in thirteenth century law texts, referring to newness in contracts. (wolterskluwer.com)
Let's face it, most of us aren't experts in thirteenth century metalsmithing. (fictorians.com)
And Finally, as it slips in to it's thirteenth year Call of Duty is still the biggest thing ever and had a record monthly active user base in November and December 2015. (thesixthaxis.com)