A calculator is a small device that does math for you. Calculators have built-in functions from simple addition and subtraction right through to complex angle manipulation functions.
You call someone when you try and get them to hear you from a long way away. You might call out by making your voice louder, or you might use a device like a phone to call them.
It is calm weather when there is no wind. In general, something is calm if it is almost still, with very little movement and excitement. The opposite of calm is excited.
A caltrop is common name given to some (unrelated) plants that have seed heads with 4 or more sharp spikes, arranged so that one of them is always pointing upwards. The seed heads are very painful to step on. Metal devices of the same name and design can be thrown onto roads in front of cars to burst their tires.
Scientific name: Dromedary: camelus dromedarius
Camels are a large animal that is very well adapted for hot, dry weather. They can survive longer without water than any other animal. Camels have a big, fatty hump on their backs that stores food for times when they can't find enough to eat. They are taller than an adult man and are used to ride on or to pull carts. There are two types of camel, dromedary camels like in the picture that only have one hump, and the bactrian which has two. There are a lot more dromedaries than bactrians in the world. Camels are related to llamas and alpacas.
Scientific name: ornamental variety: camellia japonica
Camellias are an attractive plant native to southern and eastern Asia. They have pretty flowers in shades from white through pink to deep red. They grow from about 2-20m high, and are not drought tolerant but will tolerate frost. Most people grow them as ornamental garden plants. The tea plant is a variety of camellia.
A camera is a device that is used to take photos, like the ones you see on every page in this dictionary.