A reversal is when a trend changes to a trend in the opposite direction. For example, when the upward trend of bitcoin from 2015 to 2017 where the price went from $200 to $20,000, it had a reversal when it switched to a downward trend over the next year until it bottomed out at $3,000 in 2019.

A reversal is time dependent, as are trends. You can have a short-term downward trend within a long-term upward trend. The short-term trend can undergo a series of reversals without disrupting the overall long-term upward trend.

Therefore, when talking about reversals it's important to specify the duration of the overarching trend you're talking about.