Interphase

Interphase is a phase in a cell's life that a cell spends most of its life in, there are three phases: G1, S, and G2. 

In the first phase (G1), the cell grows and takes in nutrients. In the second (S) phase, the cell's DNA is replicated. Each replicated chromosome consists of two sister chromatids connected at the centromere. 

When DNA replicates, it copies chromosomes, and when it copies it, it makes two combined chromosomes named "sister chromatids", the place of connection between the sister chromatids is named the centromere, not to be confused with the centrosome.
We call this phase the "S" phase, or synthesis, even though they are two chromatids they are still considered one chromosome.

In the last phase (G2) there's another growth phase, during this phase, the membrane of the cell gets larger and after it reaches its preferred size, the cell is then ready for mitosis.