An "allergy" is your body's negative reaction to some kind of substance, like pollen, dust, mold, or certain kinds of food. When you have contact with the substance that you are allergic to, you start to feel symptoms like sneezing, headaches, swelling, itchiness, and so on.

"Allergies" (with an -s on the end) usually means pollen allergies.

When you know that you have allergies, you may start to call them "my allergies".