What is the definition and difference between Yachid and Echad? I read some where that Maimonides told HaShem is Yachid, but isn't He Echad?
"Yachid" means alone or only like in Bereshit 22:16 and Zachariah 12:10. "Echod" means one.
Both terms are used to describe HaShem.
Rambam certainly does not deny that HaShem is one. For Rambam to say such a thing would be to deny the truth of the Torah since the statement that HaShem is one is explicitly stated in Devarim 6:4. By Rambam's own definition as found in Hilchot Teshuvah 3:8, this would be apikorsus.
The use of "Yachid" is usually applied when we try to refer to G-d prior to creation. It is to emphasize that this is how things truly are even now. This is what is expressed in Malachi 3:6, "I am HaShem, I do not change." Creation makes no change by HaShem. It is the same before creation as it is after creation. The perception of change and variation is only from the perspective of the creations.
"Echod" is used to describe HaShem's unity like in Hilchot Yesodei HaTorah 1:7 which says, "HaShem is one. He is not two or more, but one, unified in a manner which [surpasses] any unity that is found in the world; i.e., He is not one in the manner of a general category which includes many individual entities, nor one in the way that the body is divided into different portions and dimensions. Rather, He is unified, and there exists no unity similar to His in this world."
All of this is part of the reflection of the Adon Olam prayer said each day.
The Rambam, in his Hilchot Yesodei Hatorah (foundations of the Torah) writes (1:7)
אלוה זה אחד הוא--אינו לא שניים ולא יתר על שניים, אלא אחד, שאין כייחודו אחד מן האחדים הנמצאים בעולם: לא אחד כמין שהוא כולל אחדים הרבה, ולא אחד כגוף שהוא נחלק למחלקות ולקצוות; אלא ייחוד שאין ייחוד אחר כמותו בעולם.
"This God is One, not two or more than two, but One whose unity is different from all other unities that there are. He is not one as a genus, which contains many species, is one. Nor is He one as a body, containing parts and dimensions, is one. But His is a unity than which there is no other anywhere"
Translation from here, which is sourced from Louis Jacobs, "Chapter 2: The Unity of God" in A Jewish Theology (1973). Behrman House.
So the text of Maimonides uses both the word "echad" and "yachid" to describe God. One means "one" and one refers to the unique nature of God as compared to other "singular" things in the world.