If you merely added it to your Cart there will be a "Save for later" and to the right of that a "Remove" link in the bottom right of the box for that item in the Cart. You click the Remove link.
If on the other hand it is in the Cart because you committed to buy the item (removing it from the seller's inventory of items for sale) the Remove link is not there because you are obligated to pay for it. This includes if you bid and won, made an offer that the seller accepted, accepted an offer from the seller, used the Buy It Now button and you clicked to confirm that you are committing to buy the item (if the listing did not required Immediate Payment in which case you are taken directly to a payment screen when you click the BIN button, or if you used the "Request a total" link for multiple items from the same seller from within the Cart. Those items will also be in your Purchase History awaiting payment. For those you can made a request to the seller to cancel the transaction, but s/he is under no duty to do so at your request nor reply to it, and s/he can instead cancel for non-payment (giving you a strike that counts towards the ability of sellers to automatically block anyone with 2 or more strikes in the past 12 months, and affecting your buying limits) after 4 days of non-payment.
