ما الفرق بين لفظ: السلام عليكم، ولفظ: مرحبا؟

The difference between "Al-Salam Alaykum" and "Marhaban" is twofold:

First: Linguistically, "Al-Salam Alaykum" has two interpretations: 1. Al-Salam is one of the names of Allah, and the meaning is: the blessing of His name has descended upon you. 2. Al-Salam is a verbal noun meaning safety, which is the desired outcome when greeting. As for "Marhaban," its meaning is: you have met with spaciousness.

Second: "Al-Salam Alaykum" is the greeting of the people of Islam and their emblem. Initiating with it is a Sunnah, and returning it is obligatory. As for "Marhaban," it is an expression for welcoming an arrival and does not take the place of Al-Salam. A Muslim should begin their encounter with Al-Salam, and then express welcoming phrases.