A 'Trend' plot is simply a number of amplitude values - snapshots of the total vibration (vibration at all frequencies) - over a period of time. The interval between readings will be the time elapsed between those readings. That time interval could be anything from months to milliseconds depending on the specifics of the vibration program and system(s) involved.
Whereas a Trend is amplitude values versus time, a "Spectrum" Plot is amplitude versus frequency. A spectrum, a.k.a. an "FFT", allows you to assess severity (with the amplitude) and helps identify the source (with the frequency). This is the most commonly used analysis tool and is usually sufficient protection for general speed machinery.
The dreaded time domain plot - a powerful tool, but a number of questions for the analyst: How is it interpreted? How is one set up? Should I use it everywhere? Why do I need it when I have the spectrum?
The name 'enveloping spectrum' is not always a technically correct description of the signal processing but for simplicity sake will be the term we use. An enveloping spectra is the same in appearance (amplitude vs. frequency) to a conventional spectrum except for the amplitude units used.