Intel can buy a "7nm" processor from TSMC and in fact for products that aren't their processors they actually do get other companies to manufacture their products occasionally. They don't want to release the complete design of their processor to external suppliers for IP reasons and have been leveraging their manufacturing advantage to sell more CPUs. Intel could build AMD chips in their fab but then Intel CPUs have to compete with AMD on architecture alone and lose any performance edge attributable to the manufacturing.
Technically this is the gate length of the transistor (https://nptel.ac.in/courses/103106075/Courses/Lecture21.html) but it's becoming mostly a marketing term to refer to the generation of manufacturing tech since the transistor density depends on enough other factors now that gate length alone isn't a good measure. Intel's 14nm ~= TSMC/GF 10nm and Intel's 10nm ~= TSMC/GF 7nm.