A day is defined as the period of time that it takes the Earth to complete one rotation about its axis. Historically, the day was defined as the mean solar day, which is the average amount of time it takes the sun to reach its highest point in the sky twice.

Standard measures of time on earth such as days, seconds, hours, weeks, months, years, and so on, have changed over time. The second is a base unit of time in the International System of Units (SI), on which the rest of the measures of time are based on. Some of the SI base units were redefined in 2019, prior to which we've used the same definitions since 1960. Though the basis of the definition of the units has changed over time, the relationship between units is the same:

1 day  =  24 hours
   =  1,440 minutes
   =  86,400 seconds

The above breaks a day down to the base unit of seconds. Below are some of the larger measurements of time:

1 week  =  7 days
1 month  =  28-31 days
1 year  =  365-366 days

The different meanings of the term "day"

Above is the definition of a day as a unit of time. However, the term is commonly used in other ways. Some examples include:

Day as in daytime - In this case, day refers to the time during a 24 hour period where the sun is up. In other words, day is the time between sunrise and sunset, or the period during which sunlight reaches the ground on Earth, assuming nothing is blocking it. Generally, daytime lasts a little over half of the 24 hour period, though this can vary depending on factors such as time of year (season) as well as location.

Day of the week - When someone asks you what day it is, one of the ways you can interpret this is them asking you what day of the week it is. For example, the question, "What is the first day of the week?" can be answered as Monday or Sunday, depending where you are. According to the international standard, Monday is the first day of the week, but in certain countries, including the United States and Canada, Sunday is considered the beginning of the week.

Day of the month - This usage refers to the date. So when someone asks "What day is it?" you could respond with the day of the week, or you could respond saying that it is the 17th of April, 21st of September, or whatever day of the month it is.

There are many different date formats used. In the US, the date format follows month/day/year, so for 4/17/2020, the day is the second number, 17. In other countries, the day/month/year is more common. But even within the same countries, different usages exist, so it is important to confirm what format is actually being used.