a quarter

Businesses usually break the year up into four pieces, which are called "quarters". They measure the performance of the business by how well each quarter compares to the same quarter the last year.

You name the quarters like this:

The fourth quarter is always tough.

People also call them "Q1", "Q2", etc.

It's time to start looking ahead to Q3.

a spike in (a number or statistic)

A "spike" is something that's sharp and pointy.

If you're looking at a graph of numbers or statistics, and the numbers rise very suddenly, the shape on the graph also looks sharp and pointy. So when a number suddenly rises, we call it "a spike". 

Here are some examples:

There's been a huge spike in the number of deaths resulting from prescription drug overdoses.

We've seen a spike in customer complaints since we launched the newest version.

demand (for a product)

We use the word "demand" when talking about something that is bought and sold. "Demand" is how much people want something. When demand is "high", it means that a lot of people want to buy it. When demand is "low", not many people want it.

This is a term that economists use a lot. It also comes up a lot in business news stories:

Demand for nurses and other health services continues to increase.

Some say that their manufacturing output can't keep up with the demand. 

(a number or statistic) is flat

When a statistic or number doesn't change, you can call it "flat". We call it that because it looks flat when the numbers are plotted on a graph.

For example:

Sales are up, but profits are flat.

This phrase sounds professional and businesslike.