Chemistry terms can be confusing, in part because some terms have multiple related (but different) meanings depending on the branch of chemistry you're dealing with. Take for example the word substrate in chemistry: It can refer to either a chemical substrate or an enzyme substrate depending on the context that it's used in. Because the usages are related, however, learning how the term is used in different types of chemical reactions may give you a better understanding of the concept of substrates as a whole.
Chemical Substrate Definition
In chemistry, you can define substrate broadly as the medium in which your chemical reaction takes place. It's a bit more than this, however; the substrate is also typically the reactant of your chemical reaction, meaning that it is the chemical component that is actually acted upon and changed into something else by the reaction. At the end of the reaction, the original substrate reactant will no longer have the same chemical makeup.
It's worth noting that the substrate is often chemically stable before the reaction, however. In many cases, an outside chemical or application of energy is required to begin the chemical reaction; this outside influence is known as a catalyst. The catalyst begins the reaction, but is not actually a part of it; the end result will still be a change in the substrate but won't be a combination of the substrate and the catalyst.
In biochemistry, the definition of substrate changes a little. In this context, substrates are typically referred to as "enzyme substrates" and represent the organic materials that an enzyme acts upon to cause a reaction. This is similar to the reactant definition used in general chemistry, but it's important to note that this definition is a bit more narrow; it only refers to a material present in enzyme reactions, and only a certain type of material.
You might notice a similarity between enzyme reactions with the substrate and the way that catalysts begin a chemical reaction in general chemistry. In the case of biochemistry, enzymes play the role of catalyst to begin a reaction within the substrate without actually being part of the end result of the reaction.
The Core Concept of Substrates
Despite the slight differences in the general chemistry and biochemistry definitions of substrate, the core concept should be pretty clear. As far as chemistry is concerned, a substrate is generally seen as a chemical material that some other material can act upon to cause a change. The change occurs to the substrate itself and not the outside catalyst or enzyme, and in most cases it could likely happen on its own if enough time were allowed.
As with the more specific definition seen in biochemistry, other niches of chemistry may also have specific definitions of the word "substrate" that differ a bit from the general definition. The core concept will remain the same, however, regardless of the specifics imposed by the niche. While the context and details may differ, substrates in chemistry will always be some form of chemical or molecule that another chemical or object can act upon in some way.
Substrates in Other Sciences
Keep in mind that chemistry isn't the only science that makes use of the term "substrate." Biology uses the term to indicate a growth material for biological organisms (such as the material that bacteria grow on in a petri dish), while geology defines substrate as the underlying layer of rock or other materials found beneath the soil. Other sciences such as materials science also use the term with slight variations on its meaning as well. While the specifics differ from one science to another, however, the word substrate is generally defined as some sort of core or surface throughout the scientific world.