I'd like to know how to make sentences using "downtime". I came across this word for the first time today and when I looked up in the dictionary, the meaning is almost the same as "free time" or "spare time". Am I correct? Then is it all right if I say the followings? In my downtime I like to go cycling. I need some downtime next week. I also like to know if I can use this word interchangeably with free time or spare time. Is this word very common in daily conversation?
I wouldn't use downtime at all in a "personal" context. As this NGram shows, in recent decades some people have transferred the "office jargon" term for non-operational through not working to free...
I would definitely say that "downtime" is commonly used. It will make sense if it is used interchangeably with "free time" or "spare time", but it generally implies relaxation, rest or a halt in activities.
If you were to say "In my downtime I like to go cycling" then it would imply that for you cycling is relaxing. The phrase "In my free time I like to go cycling" doesn't carry this implication.
I'm going to say that the answer to this question is sort of a combination of harrietgrace and mustafa's answers.
The vast majority of the time, the word downtime is a negative thing meaning that a system is not working (often "network downtime" as mustafa mentioned).
On the other hand, the word can also occasionally mean something like "free time." Normally, I pronounce the word as two words when using it in this sense. I am not sure whether it is properly spelled "downtime" or "down time." In this case, I somewhat disagree with the idea that what you do during your downtime must be relaxing to you. Merriam-Webster.com's second definition parenthetically indicates that the word applies in-between periods of work. I think this is more or less consistent with the way the phrase is used in practice. For example, someone who works two jobs - one in the morning and one at night - might have a couple of hours of downtime between jobs. That person might spend that time paying bills, doing taxes, napping, etc.