user33701: What I'm doing wrong or why is it not working: 
#define Uin A0;

void setup() {
}

void loop() {
  analogRead(Uin);
}

Results in a Error (expected ')' before ; token. 
I thought #define just exists as a place holder.

user4143: Because you have a ; at the end of your #define.
#define Uin A0;
analogRead(Uin);

becomes:
analogRead(A0;);

The Arduino website mentions this in their reference. For more in-depth detail about the C pre-processor you can read the (rather large) manual here.

33701: <p>What I'm doing wrong or why is it not working: </p>

<pre><code>#define Uin A0;

void setup() {
}

void loop() {
  analogRead(Uin);
}
</code></pre>

<p>Results in a <code>Error (expected ')' before ; token.</code> </p>

<p>I thought <code>#define</code> just exists as a place holder.</p>
 4143: <p>Because you have a <code>;</code> at the end of your #define.</p>

<pre><code>#define Uin A0;
analogRead(Uin);
</code></pre>

<p>becomes:</p>

<pre><code>analogRead(A0;);
</code></pre>

<p>The Arduino website mentions this in <a href="https://www.arduino.cc/en/Reference/Define" rel="noreferrer">their reference</a>. For more in-depth detail about the C pre-processor you can read the (rather large) manual <a href="https://gcc.gnu.org/onlinedocs/cpp/" rel="noreferrer">here</a>.</p>
