However, these days, for many people, the term "meme" has taken on a narrower meaning. It has come to be used as an abbreviation for "internet meme" - which is a term most frequently used to refer to a piece of highly viral (but usually rather frivolous) culture which is shared frequently on the internet.
As Tom Michael put it:
It's ironic that the term "meme" has spread much more widely as a term relating to internet silliness than as a general unit of cultural exchange. It had to mutate into a faster-reproducing form in order to be widely known about.
Sometimes the frivolous internet culture is described as being a "meme", as it is being passed around. In such cases, the term "meme" has become a kind of meta-meme - which spreads due to its association with the viral culture which it is associated with.
In biology, there is the concept of "linkage". This causes the fate of genes to be associated with the fate of their neighbouring genes. Some genes can us this effect to spread - because they are near other genes which are favourably selected for. This is commonly known as genetic hitchhiking.
The term "meme" is doing some hitchhiking of its own these days, spreading by virtue of its association with the highly-viral content which it has come to be associated with.