To estimate something is to guess at an amount.
You can use this word as a verb, a noun, or as an adjective. In these sentences, "estimate" is a verb:
My mechanic estimated the total costs for repairs on my car would amount to $350.
How much time do you estimate it will take to walk to the nearest grocery store?
Doctors estimated that the man had only a few more months to live.
A real estate appraiser estimates the value of a person's house. (This information is necessary prior to the purchase or refinancing of a house.)
When this word is used as a noun, the vowel sound on the third syllable changes: estimate. Do you hear the difference? As a verb, you pronounce the word as estimate. As a noun, you pronounce it estimate. (a short "i" sound.)
How much was the estimate?
The estimate came to $350.
We need an estimate on some plumbing work.
Can I get an estimate?
You can also use "estimation" as a noun. In this case, there can be a few different meanings for the word:
The estimation for the work on my car was kind of high.
What is your estimation of the political situation in Egypt?
As an adjective, the word changes slightly to "estimated."
Our estimated time of arrival is 10:30. (A popular abbreviation for this is "ETA.")
The estimated costs were much higher than the actual costs for the repair work.