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In this concept, we're going to learn about basic geometric definitions.

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This is so that we can make sure we know the basic vocabulary

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that will help us to be successful in geometry.

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The first word you need to know is a point.

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And a point is basically just like a dot in space.

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You've probably heard the word point before.

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The main thing you need to know about a point is that,

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technically, in math, it has no length, width, or height.

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So you can't measure it.

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This is what a line looks like and, by definition,

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a line is straight and goes on forever.

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So that's why you put the arrows at the end,

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to indicate that it keeps going past where I stopped drawing it,

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all the way over there and forever and ever.

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If we want to have our lines stop at a certain point,

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like maybe it goes on forever in this way,

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and then stops here, this is now called a ray,

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when it only extends in 1 direction.

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If we want to have it stop in both directions,

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it'll look like this, and this would be called a line segment.

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Those points that stop the line are called endpoints.

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So a ray has 1 endpoint, and a line segment has 2 endpoints.

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Any time you have a line that has points on it,

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Those points are called collinear because they are on the same line.

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And the word collinear has this prefix, 'co,'

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which means same, and you see 'line' in here.

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So they're on the same line.

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If I had another point over here, it would not be collinear with these

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first 3 points, because it is not on the same line.

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Now, the last basic word term that we're going to talk about is plane.

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And a plane is basically a 2-dimensional surface

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that extends on forever in all directions.

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It's sort of hard to draw, but if you think about a piece of paper extending forever,

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like a piece of paper that goes on forever, that would be a plane.

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You've actually heard the word plane before, probably in algebra.

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You have the normal coordinate plane as your xy axes.

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This is a plane because it goes on forever.

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The plane is this whole surface right here, where all the points lie.

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And you know that there are points, infinite points that go on forever.

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If you have points that are on the same plane--like any points

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on the xy coordinate plane would be considered coplanar

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because they're on the same plane.

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So you might think, well, how could there be

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points besides the ones on the xy plane?

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Well, you could have one above it.

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Now, that's hard for me to draw, but you could have

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a 3-dimensional shape, and then you could have

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multiple points that are not on the same plane.

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So I'm going to draw a cube here, and on this cube

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these points, they're all on the front face of the cube, are coplanar,

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but the point over here on another face is not coplanar

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because it's not on the same plane as this front surface

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if we were to extend that front surface in all directions.

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Now, the last thing we want to talk about is 2 words: postulate and theorem.

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And you'll see these words a lot in geometry.

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They mean similar things.

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A postulate is something that we assume is true,

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and a theorem is something that we have to prove is true.

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So 1 example of a postulate would be

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if we have 2 lines that we know intersect,

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and intersect means cross each other,

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then those 2 lines have to intersect in a point.

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This is a postulate because it's not something that we're going

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to prove, we just sort of assume that it's true.

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There's a similar postulate that you should know about planes.

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If you have 2 planes and you know they intersect,

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then they have to intersect in a line.

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And that's something that you could look--

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visualize by looking back at this cube.

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These planes, like the top face of the cube

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and the front face intersect in 1 place:

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They intersect in this line.

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All right. At this point, you should look at the next video,

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which will go through some examples.