Deep describes something that extends far down from the top or far in from the outside. A deep swimming pool might go down twelve feet instead of four, giving divers room to plunge safely. A deep cave stretches far back into a mountain, its passages winding beyond where flashlights can reach. When you dig a deep hole in your backyard, your shovel travels a long way before hitting hard clay or rock.

The word also describes ideas, feelings, or thoughts that go beyond the surface. A deep conversation with a friend explores meaningful topics like hopes and fears, discussing what matters most to you both. A deep book makes you think hard about life's bigger questions. When someone says “that's deep,” they mean an idea has layers of meaning worth exploring.

Deep can describe intensity too. A deep sleep is so sound that thunder might not wake you. Deep trouble is serious, not just a minor problem. A deep color is rich and saturated, like the deep blue of the ocean far from shore. Deep breathing fills your lungs completely, drawing air down rather than taking shallow breaths.

The opposite of deep is shallow. A shallow stream barely covers your ankles, while a deep river might be over your head. Shallow thinking stops at the obvious answer; deep thinking explores further.