According to the Qur'an, the authentic Sunnah, and the understanding of mainstream Salafi scholars (such as Muhammad ibn Salih al-Uthaymeen, Abd al-Aziz ibn Baz, and the scholars behind IslamQA), a mahram is:
A man whom a woman is permanently forbidden to marry because of blood relationship, breastfeeding (rada'ah), or marriage (musaharah). ([Islam-QA][1])
This definition excludes men whom she is only temporarily forbidden to marry (such as her sister's husband or a married man), because they could become lawful to marry in the future.
These are permanent mahrams.