What legal malpractice is, and what it is not
Legal malpractice is professional negligence by a lawyer: a failure to exercise the knowledge, skill and care that a reasonably competent lawyer would exercise in the same situation, which causes the client a loss. The measure is the ordinary, careful lawyer, not the finest one available. The ABA Model Rules describe the professional standard in general terms, with Rule 1.1 requiring competent representation and Rule 1.3 requiring reasonable diligence and promptness. Those rules govern discipline, not lawsuits, but courts often let juries hear them as evidence of what a careful lawyer does.
The most common fact patterns are procedural. A lawyer lets the statute of limitations run without filing, misses a court deadline and the case is dismissed, settles without authority, names the wrong defendant, or gives wrong advice on a contract or a will. Legal malpractice can also be an intentional wrong, such as taking money from a trust account or breaching a fiduciary duty, and some states treat those as separate claims with their own rules.
What legal malpractice is not: a lost case, a verdict smaller than you hoped, or a strategy that did not work. Lawyers make judgment calls, and a reasonable judgment that turned out badly is not negligence in most states, sometimes called the attorney judgment rule. Rudeness, slow communication and a large bill are grounds for a bar complaint or a fee dispute, but they support a malpractice claim only if they caused a provable loss.