MUHADDITH OR MUHADDIS
Muhaddith (Arabic: محدث) generally refers to a Hadeeth Scholar.
The term muhaddith refers to a specialist who intensely knows and narrates hadith, the chains of their narration isnad(Sanad), and the original and famous narrators. The requirements for a muhaddith are the same requirements that apply to the reception and transmission of reports (riwayah) in the Islamic tradition more generally: truthfulness, integrity, a competent and accurate memory, being free of prejudice or compulsion that might be presumed to distort the reporting.
Muhaddith is someone who knows hadiths and their chain of transmissions [tariq], names of narrators, and the various wordings [matan]. Technically, the term ‘muhaddith or Muhaddis’ can refer either to a transmitter of hadith or a collector or the compiler of ahadith.
Al Taj Al Subki said: The Muhaddith is he who knows the chains of narrators and defects, the names of narrators, the high chains and low chains, and memorized numerous matan and heard from scholars the six books of Hadeeth in addition to Musnad Ahmad, Sunan Al Bayhaqi, Mujam Al Tabarani, and on the top of all that he memorized 1000 juzi of ahadeeth. This is the least rank and if he heard from scholars all what I mentioned and wrote the tibaq and studied under scholars of hadeeth and taught defects of hadeeth, deaths of narrators, and masaneed he will be in the first rank of Muhaddith, and Allah increase in knowledge whoever He wants."
Al Zarkashi quoted Abu Al Fath, Ibn Sayyid Al Naas saying: "The Muhaddith is he who works in the science of hadeeth; riyawah, diraya and kitabah, and knows numerous number of narrators and narrations and was well versed in them until he memorized all that become famous with his accuracy in them."
Al Ramharamzi (D. 360H) stipulated an age condition so one be qualified to such rank that he is no younger than 50 years old and he quoted many athaars from the scholars of pious predecessors supporting this. 
Muhammad ibn Haatim ibn al-Muzaffar (may Allaah have mercy on him) said: Allaah has honoured this ummah and favoured it over others by blessing it with the isnaad. No other nation has this blessing, and they do not distinguish between that which was revealed in the Tawrah and Injeel and was brought by their Prophets, and that which was added to their books of narrations transmitted from inauthentic sources.
This ummah narrates hadeeth from a trustworthy individual who was known at his own time for sincerity and honesty, from another of similar character, and so on until the end of the chain of narrators. Then they researched very carefully to find out who had the stronger memory and was more precise, and who spent more time with the one from whom the report was transmitted, and who spent less time, then they would write down the hadeeth from more than twenty chains of narration, so that they could be sure that they had eliminated any mistake or error from it, and they wrote it exactly as it was narrated.
This is one of the greatest blessings that Allaah has bestowed upon this ummah. We ask Allaah to inspire us to thank Him for this blessing and we ask Him to make us steadfast and to guide us to that which will bring us closer to Him and make us adhere to obedience to Him. End quote. Sharaf Ashaab al-Hadeeth