What are trichomes and what are their functions?

Trichomes are hairs which are often found as epidermal outgrowths. The hairs of aerial parts may be unicellular or multicellular, branched or unbranched.
Function : (a) They enclose stationary air and protect the plant organ against water loss and sudden fluctuation in atmospheric temperature.
(b) In some plants, the hairs are glandular which secrete essential oil and provide characteristic odours to the plant, e.g., Citrus, Ocimum etc.