means A general direction in which something is developing or changing, often one that catches on widely for a time.

from From Old English 'trendan,' to turn or roll, kin to a family of Germanic words about rotating and revolving. For centuries it kept that literal sense — rivers and coastlines were said to 'trend' this way or that, meaning they bent in a direction. The figurative leap to fashions, opinions, and fads (the way things are 'turning') is a more modern development, and the buzzword 'trending' is a child of the social-media age.