The most beautiful and oldest definition of a companion (sahabi) is:

“A companion of the Prophet Muhammad, peace be upon him, is the one who enters Paradise with him !”

This definition is intoxicating. For it not only includes the good people who followed the Prophet Muhammad, peace be upon him in his time, but it also includes all the good people who follow him afterwards till the end humanity on this earth. And obviously, it excludes all the evil people in his time and afterwards.
The Prophet Muhammad, peace be upon him died in year 11 Hijra / ~632 CE.

This definition comes from Salman, the Persian. He is counted as one of the People of the House of the Prophet Muhammad, peace be upon him.
He learnt from one monk after another, till he was freed by the Prophet Muhammad, peace be upon him, and he accompanied him for seven years.

This oldest definition is found in a sound report in Hilyatul Awaliya (Ornament of the Holy People) by Abu Nuaym Al-Asbahani and Tarikh Dimashq (History of Damascus) by Ibn Asakir. And here is the report:

“Al-Ash’ath Ibn Qais [He became a Muslim in 10 Hijra / ~631 CE] and Jarir Ibn Abdillah Al-Bajali [He became a Muslim in 11 Hijra / ~632 CE] came to Salman [He became a Muslim in 5 Hijra / ~627 CE], may Allah be pleased with them, and they entered upon him in a hut in the district of Al-Mada’in (few kilometres south of Baghdad in Iraq). They came to him, greeted him, and then said: Are you Salman Al-Farisi? He said yes! They said: Are you the companion of the Messenger of Allah, may Allah bless him and grant him peace? He said: I don’t know. They became suspicious and said: Perhaps it is not what we want. He said to them: I am the companion you want, and I saw the Messenger of Allah, may Allah bless him and grant him peace, and I was sitting with him, but his companion is the one who enters Paradise with him, so what do you need? They said: We came to you from a brother of yours in the Sham (Syria, Lebanon and Palestine). He said: Who is he? They said: Abu Darda. He said: Where is the gift he sent with you? They said: He did not sent with us as a gift. He said, “Fear Allah and fulfil your trust. No one comes to me from him without bringing with him a gift.” They said: Do not raise this against us, for we have wealth, and you can dispose of it according to your will . He said: I do not want your money, but I want the gift that he sent with you. They said: No, by Allah, he did not send anything with us! However, he said that there is a man among you who, when the Messenger of Allah, may Allah bless him and grant him peace, was alone with him, would not seek anyone else, so if you come to him, extend my greetings to him. He said: What gift did I want from you other than this? What better gift than peace, a blessed and kind greeting from Allah.”

Both Al-Ash’ath Ibn Qais and Jarir Ibn Abdillah Al-Bajali met the Prophet Muhammad, peace be upon him. However, they did not strictly see themselves as companions of the Prophet.

As to Salman, not only his heart was with the Prophet Muhammad, peace be upon him, but it transcended the boundaries of time to reach all the good people as they enter Paradise in the company of the Prophet Muhammad, peace be upon him.