These two words are often used interchangeably, and there's actually a good reason for it. However, they should be different.
Altitude - measured height above the ground Elevation - measured height from mean sea level
Specifically, if you were at an elevation of 1000m, and you throw a ball 5 metres in the air, the ball is at an altitude of 5 metres. If you throw that ball off of a 200m cliff, it is at an altitude of 205m, over ground that is 800m elevation.
The reason everything gets messy, and why these two words have officially become synonimous, is that "Altitude" as it relates to aviation is measured height from sea level at all times. This had to do with measured barometric pressure, etc., but one can land a plane at 400m altitude as long as the runway is at 400m elevation.
The definition of these words is often used interchangably, but they are considered different for specific reasons. An interval is casually recognised as the "time between things". That's actually the duration. Duration is how long any activity lasts. Why does this matter? Think of it this way... the duration of a full moon is only a few days, but the interval is 29 days.
What might be seen as interchangeable here definitely is not. Coordinated Universal Time is a measurement that never varies. UTC is always UTC. There is no Daylight Saving or British Summertime.
GMT, or Greenwich Mean Time, is a time zone. London sits within this time zone. It is a poorly named time zone because at one time in history, GMT was the time which everything was based simply because of where it existed.
Today, interchanging these two can be disaster when trying to describe time. UTC is a measured time, and GMT is a time zone.