Al-Qaamus al-Muhit is the work of a prominent Sunni scholar and linguist; he defines sahabah as “al-muasharah wa al-mulazimah,” which means living together or associating together inseparably. Raghib al-Isfahani says, “This term applies only to the one who is constantly and continuously in companionship (with another person).” Therefore, according to these definitions, a companion of the Prophet would be someone who associated very closely with him, regardless of whether he was a Muslim or a non-Muslim, righteous or unrighteous, and whether he believed or disbelieved in him.
However, Islamic jurists (usuliyun) unanimously agree that for someone to bear the title of “companion,” he or she must have been Muslim and must have accompanied the Prophet for a long period (tallat mujalasatuhu), while listening attentively to him and learning from him, not just merely visiting him or learning from or about his knowledge.
The Muhaddithun (school of Narrators) defines “companion” as being “every Muslim who saw the Prophet.” Some scholars define it as, “any Muslim who lived during the time of the Prophet even if he did not see the Prophet.” Still other narrators further expanded the definition of “companion” to encompass every Muslim who met the Prophet and believed in him, and then apostatized and then reverted to Islam.
 Al-Mufradat fi Gharib al-Qur’an, the section on Sahab
 Miqbaas al-Hidayah; Al-Darajat al-Rafia, 10
 Mukhtasar, 2:67
 Miqbaas al-Hidayah
 Ibn Hajar al-Asqalani, 1:10