Ijtihād
اجتهادA qualified scholar's disciplined effort to derive a ruling from the sources.
Definition
Ijtihād requires mastery of Arabic, the Quran, hadith and legal theory — it is not a licence for personal preference. Where the sources are decisive there is no ijtihād; it operates in the space of probable evidence. This is why the four schools can differ without any of them leaving Sunni orthodoxy.