A ṣaḥābī (صحابی), Companion, is a person who:
• met the Prophet ﷺ;
• believed in him at that encounter;
• and died upon faith.1
Seeing with one's eyes is not required. A blind person who met him as a believer and died upon faith is also a Companion.
1. Plural form: Ṣaḥābah (صحابہ) is also used for several Companions.