A trend is what's hip or popular at a certain point in time. While a trend usually refers to a certain style in fashion or entertainment, there could be a trend toward warmer temperatures (if people are following trends associated with global warming).
A trend simply reflects what seems to be going around at any given time. A trend can be in any area and doesn't only reflect fashion, pop culture and entertainment. There can also be a trend in the stock market to be bullish or bearish, depending on economic indicators, or a political trend reflecting a nation’s current mood. Some trends are fun, some fabulous, some appalling, but however long they last, you can be sure there will always be a new trend coming along to replace the old.