Over time words change their meanings, and their meanings can change in several ways:

• A particular meaning of a word can be used less and less often to the point that many people don't know the old meaning. For instance, “charity” in English used to mean “love of one’s fellow man,” a meaning of the word that nowadays is not well known outside of religious circles.
• A general meaning of a word can be used more often in a more specific sense. “Charity” is again a good example, which nowadays mostly means “giving money to the needy” or “an institution that helps the needy.”
• A particular meaning can be generalized.
• A brand new meaning can be applied to a word with only the vaguest of connections to its previous meanings. The use of “mouse” in the sense of a device used to reposition the cursor on a computer screen.

And when a word is being used in a new sense, that sense is not usually reflected in dictionaries for some time. One such word that has a very common meaning these days that is not shown in most dictionaries is the word акция which can now mean "sale, special" in the sense of a temporarily reduced price. I was very surprised to learn this meaning this summer, because previously the most common meaning of акция was “a share of stock.”

Here are some examples of that usage from this summer. In this first ad if you buy малютка, then you get a free bib, so probably the best translation of акция is “special”:

In this next one if you buy Ryaba brand mayonaisse, you can win 10,000 rubles, so again “special” is the best translation:

This next one is a banner hung in front of a Пятерочка grocery store. Notice they've added супер to the word, so here we have a supersale: