When talking about adding fractions, we say, "You need a common denominator".
There are many common denominators; it's best to use the smallest one, to make the arithmetic simpler.
That number is the Least [of the] Common Denominator. In other words, we call it the LCD.
Denis is correct, also. When finding that number, we need to consider lists of multiples of each denominator, and look for values common to both lists. The LCD is the smallest multiple that's common to both lists".
So, that number is also called the Least [of the] Common Multiple. In other words, we also call it the LCM.
I think both descriptions are common.
lookagain gave the easy formula. Divide the product of the two denominators by their GCF.
tkhunny gave the way to their GCF: examine the prime factorizations of the two denominators.
EG:
Let's add the following two fractions.
1/90 + 1/140
We need a common denominator. Let's find the smallest one (the LCD).
Begin by examining the prime factorization of each denominator.
90 = 2 * 3^2 * 5
140 = 2^2 * 5 * 7
The factors common to both numbers are 2 and 5.
(2)(5) = 10, so 10 is the Greatest Common Factor (GCF) of 90 and 140.
Multiply the two numbers together, and divide by their GCF.
(90)(140)/10 = 1260
1260 is the Least Common Multiple (LCM) of 90 and 140; therefore, 1260 is the Least Common Denominator (LCD) for adding those two fractions.
1/90 + 1/140
Switch to the common denominator:
14/1260 + 9/1260
Now we can add them because the denomiators are the same:
(14 + 9)/1260 = 23/1260
We just used the LCD to add 1/90 to 1/140. Their sum is 23/1260.