In mathematics, the lowest common denominator or least common denominator (abbreviated LCD) is the least common multiple of the denominators of a set of vulgar fractions. For instance, the LCD of {1/2, 1/4} is 4, while the LCD of {1/2, 1/3} is 6.

By using this (or any multiple of it) as a denominator, it enables addition, subtraction or comparison of fractions:

• 1/2 + 1/4 = 2/4 + 1/4 = 3/4;
• 1/2 − 1/3 = 3/6 − 2/6 = 1/6;
• 1/3 < 2/5 since 5/15 < 6/15.

In non-mathematical use, the term is used figuratively to refer to the "lowest"—least useful, least advanced, or similar—member of a class or set which is common to things that relate to members of that class. For instance, ASCII characters are the lowest common denominator for computers, in that this set is very limited, but practically every modern computer can interpret binary data into these characters.

In the context of mass media, the term is commonly used to refer to a target audience. The implication is that the media in question are targeted at the lowest expectations of an audience with a varying range of tastes. In this regard, it is often also used to mean "dumbest people", or those whose tastes match the lowest values of the wider audience. Action movies and most television programming are often considered to be targeted at the lowest common denominator of the population. It is also used to describe negotiations and agreements which only cover the points where everybody's previous positions coincide. Highest common factor may be a more accurate description, but would not have the same negative sound.