This study examined 283 online learner dictionary definitions in terms of scores based on word frequency level and readability. Results revealed three findings. First, in terms of word frequency levels, definitions from the Cambridge learner dictionary incorporated fewer non-high frequency words (mid and low frequency words) compared to Oxford, Dictionary. com and Collins COBUILD learner dictionaries. Second, in terms of readability, definitions from the Random House learner dictionary were written at a significantly higher grade level compared to the other six online learner dictionaries. Third, in terms of both level of frequency words incorporated into definitions and readability grade level definitions, the Cambridge, Merriam-Webster and Longman online learner dictionaries were easier to understand compared to online learner dictionaries from Dictionary.com, Collins COBUILD, Oxford and Random House. Overall, the findings suggest both word frequency level and readability might contribute to the difficulty of online learner dictionary definitions.