Spanish translation: Predeterminado o preestablecido
Login or register (free and only takes a few minutes) to participate in this question.
You will also have access to many other tools and opportunities designed for those who have language-related jobs (or are passionate about them). Participation is free and the site has a strict confidentiality policy.
Explanation: I find this preferable for three reasons: it translates both the adj. "default" and the adv. "by default", it's shorter, and it works in a lot of contexts "predeterminado" doesn't. Think about a phrase like "new default values".
"Por omisión" is also used.