Trend
A trend is the dominant direction of price. An uptrend makes higher highs and higher lows; a downtrend does the opposite.
Dow's classic definition is still the most useful: an uptrend forms ever higher peaks and troughs, a downtrend ever lower peaks and troughs. When neither appears, the market is ranging.
A trend depends on the timeframe. EUR/USD can be bearish on the daily chart and bullish on H1 during a bounce. Always start with the higher timeframe.
Trading with the trend generally improves your odds, because you are going with the dominant flow. The end of a trend is often signalled by a break in structure: a lower low in an uptrend, for example.