Forgetting a new word the day after you meet it isn't a sign you have a bad memory. It's the default. What separates a word that sticks from one that fades is what you do in the days right after you first meet it.
Test yourself before you check the answer
Before looking up a word's meaning again, try to recall it first, even if you get it wrong. The effort of trying to remember, called active recall, builds a stronger memory than passively re-reading the definition. Getting it wrong and then seeing the right answer still teaches you more than skipping straight to the answer.
Build a mnemonic
A mnemonic ties a new word to something already familiar: a sound-alike phrase, a small image, or a link to a word you already know. The word meticulous, meaning careful about details, might stick better if you picture someone meticulously counting each tick on a clock. The link doesn't need to make sense to anyone else, it just needs to make sense to you.
Space out your review
Reviewing a new word five times in one sitting teaches you far less than reviewing it once today, once in three days, and once again in a week. Each gap where you nearly forget, then successfully recall, strengthens the memory more than an easy repeat right away.
Chunk related words together
Words that share a root, a theme or a sound are easier to hold onto as a group than as separate items. Learning a handful of related words in one sitting, rather than an unrelated list, gives your memory more hooks to hang each one on.
Use it, don't just review it
A word you've written or said out loud in a real sentence is more memorable than one you've only reviewed on a flashcard. Try working a new word into a text message, a sentence in your notes, or a conversation within a day or two of learning it.
Practice with a game instead of a list
Testing yourself doesn't have to look like studying. A matching round or a quiz forces the same active recall a flashcard does, just with a timer or a score attached. Tile Streak's vocabulary matching game and vocabulary quiz both work this way, and the word of the day gives you one word to actively recall the next time it comes up, rather than a whole list at once.
For the broader picture beyond memory specifically, including reading habits and daily routines, see how to increase your vocabulary.