Definition: A particular, minute unit of mass, defined differently for different substances, but so that varying substances of the same general type have the property that one international unit of the one has the same effect on the human body as one international unit of the other.

Alright class, settle down, let’s take a look! Now, you've come across the word "units," and the dictionary gave you quite the description – it’s a mouthful, isn’t it? Let’s break it down nice and slow. Essentially, “units” are like little building blocks for measuring things. But they aren't just any building blocks! They're special ones that work in a very consistent way. Think of it this way: Imagine you have different types of apples – Granny Smiths, Fujis, Galas - all apples, but with slightly different weights and sweetness. We wouldn’t measure them all with the same unit of weight, would we? We'd use grams or ounces! But some things, like vitamins or hormones, are so important that scientists decided to create a special “unit” for them. This "international unit" – which is part of what the dictionary explains – means that one IU (International Unit) of that specific thing will always have the same effect on your body as another IU of the same thing . It’s like saying, “One scoop of this powder has the same power to help you feel better as any other scoop!" Key takeaway: "Units" are standardized measurements – they're consistent within a specific type of substance. They let us compare and understand how different amounts of something affect us in a predictable way. Does that make sense? Any questions about the idea of “units” and how they’re used to measure things like vitamins or hormones? Don't be shy!