From OFLB

Sometimes during informal conversation, people refer to "the public domain" as works that are available to the general public. This is okay in conversation, but legally, when a work is in the public domain that means copyright has expired for that work.

In some countries, such as the USA, you can place your font in the public domain with a Creative Commons Public Domain license declaration.

However, the act of placing things in the public domain is not recognised in all countries, so we recommend using the Open Font License instead.

If you are thinking of dedicating your font to the public domain, please discuss this on our community mailing list.

We also have a page discussing when typeface designs enter the public domain.