The full cost of a thing across its life, covering acquisition, running, change, support and eventual retirement, rather than the purchase price alone.
Total cost of ownership counts everything an organisation will spend on something from the decision to acquire it to the day it is switched off. That includes the obvious acquisition figure, and also hosting, support, licence uplifts, the people kept on hand to run it, the price of every change made after go live, and the cost of migrating off it at the end. The phrase whole life cost is used interchangeably in UK public sector guidance. Comparing a purchase price against a build estimate is the most common way a sourcing decision goes wrong, because only one of the two figures is a total.
See also
Where this comes up
A delivery manager does not need to know contract law. They need to notice the moment a delivery problem becomes a contractual one, because after that moment informal handling starts costing money.
A make or buy analysis compares the whole life cost of building something against buying it, and finds the point where the cheaper option changes. Capability, control and risk usually decide it.