A wiki page says
A prime meridian is the meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°.
which uses the phrase "is defined to be".
Are these two phrases interchangeable? If no. When to use which? Could someone please give a hint? Thanks in advance.
Archived: there was a typo in my previous search on ELL. The Google Ngram has alway been the same.