Wikipedia
Usulis are the majority Twelver Shi'a Muslim group. They differ from their now much smaller rival Akhbari group in favoring the use of ijtihad (i.e., reasoning) in the creation of new rules of fiqh; in assessing hadith to exclude traditions they believe unreliable; and in considering it obligatory to obey a mujtahid when seeking to determine Islamically correct behavior.
Since the crushing of the Akhbaris in the late 18th century, it has been the dominant school of Twelver Shi'a and now forms an overwhelming majority within the Twelver Shia denomination.
The name Usuli derives from the term Uṣūl al-fiqh (principles of jurisprudence). In Usuli thought, there are four valid sources of law: the Quran, hadith, ijma' and 'aql. Ijma' refers to a unanimous consensus. Aql, in Shia jurisprudence, is applied to four practical principles which are applied when other religious proofs are not applicable: bara'at (immunity), ihtiyat (recommended precautions), takhyir (selection), and istishab (the presumption of continuity in the previous state).