E is the fifth letter in the English alphabet, representing a vowel sound. It is one of the most commonly used letters in the English language and is often found in the endings of words to indicate various grammatical functions such as past tense or plural forms. Additionally, 'E' is used in scientific notation to denote 10 to the power of a given number, as in '2.5E6' which means 2.5 multiplied by 10 to the power of 6.