What was a 'day' in Genesis chapter 1?

The word 'day' in these verses is subject to much interpretation. A few things to consider:

1. Are boundaries  set by 'evening and morning' in each verse where day is used?

2. By examining the word 'day' in a concordance, does it ever refer to a longer period of time?

3. When accompanied by enumeration (1,2,3 or first, second, third etc.), are we talking about a unit?

4. In Genesis 1:18 God says the sun rules the day and stars and moon the night. Doesn't this define 'day' by God's standards?

Biblical research might intimidate some people.  The more I work the Word, the greatest value comes with just reading it.  It is great to have keys to research the Bible and see its preserved integrity. I have found that God usually has the answer right in the verse, or in the context or where it's been referred to before in the Word. It also helps to understand the biblical timeline of events and the location of a particular concept (i.e. before Jesus Christ or after Jesus Christ's death resurrection and ascension.) in that timeline. The 4 gospels period, when Jesus Christ was ministering to Israel is certainly a different time than the time when the first century church reached out to the Gentiles. 

Biblical research starts with simply reading the Word.