God’s Yichud is one of Ramchal’s foremost themes , hence Klach launches into a discussion of it from the first, where it speaks of it in terms of God’s sovereignty and omnipotence . But Ramchal really doesn’t offer a definition of the term, which doesn’t simply mean “one” as in its cognate term echad. He does define Yichud elsewhere in his writings, though.
When challenged to define it in Da’at Tevunot with the query that doesn’t “Yichud (simply) refer to the fact that God alone is truly one?” Ramchal offers that “while that’s true in broad terms” even so the expression means a lot more . “It isn’t enough to say that God is unique in His being when we say He’s ‘one’,” he offers, “with the implication that only His existence is imperative , and that He’s the only Creator . We’d also have to understand that He’s the sovereign Ruler, that He alone holds absolute sway over everyone and everything else in creation, and that nothing and no one can thwart His actions or wishes — which is to say that His sovereignty is utterly supreme (i.e., “one of a kind”) .
Not only is all that so, but it’s likewise true that “He alone oversees everything personally, and (that) everything that comes about in the world only occurs because He wills it and brings it about rather than by happenstance, by (the laws of) nature, or by the (influence of the) constellations. Instead, He serves as the Judge (i.e., Ruler) of the world and everything therein, and directs everything that’s done above and below to the very last measure” . Consider the ramifications of all that, if you will!
But as we’ll see, there’s even more to be said about this.
 As R’ Shriki puts it, “every single student of Ramchal’s writings knows that one of the fundamental themes that occupied (him) practically his entire life was the issue of God’s Yichud” (Rechev Yisrael p. 167).
See Ramchal’s opening comments to Petach 1 that “God’s Yichud is the foundation of (the Jewish) faith and the root of wisdom, so it must be explained first”. Also see Ma’amar HaVichuach p. 45.
 He says that the implications of the term Yichud is that “only His will functions”, that “He alone reigns” (Petach 1), that “there’s no sphere of influence apart from Him” (Petach 2), and “that He has neither deterrents nor defects” (Petach 4). And he also adds quite mysteriously that God’s Yichud will revealed eventually (Petach 4), whatever that means (see below).
 Da’at Tevunot 35. Though he doesn’t say as much, Ramchal is alluding to the way earlier commentators defined the term, including Rambam’s statements in Hilchot Yisodei HaTorah 1:1, 7, Ibn Pakudah’s in Chovot HaLevovot 8:3 as well as in the “Invocation” at the end, and others.
 That is, since nothing else could exist if He didn’t, His existence is absolutely necessary, which is unique to Him. Everything else is dispensable. See Derech Hashem 1:1:3.
 See Derech Hashem 1:1:6.
 That is, that His sovereignty is “one of a kind” that way, as in echad and Yichud. Da’at Tevunot 35. Also see 40 there (p. 26) and 46 (p. 37).
 Da’at Tevunot 36 (p. 16).
(c) 2010 Rabbi Yaakov Feldman
Feel free to contact me at email@example.com
AT LONG LAST! Rabbi Feldman’s translation of Maimonides’ “Eight Chapters” is available here at a discount.
You can still purchase a copy of Rabbi Feldman’s translation of “The Gates of Repentance” here at a discount as well.
Rabbi Yaakov Feldman has also translated and commented upon “The Path of the Just” and “The Duties of the Heart” (Jason Aronson Publishers).