Strategies used by corporations to discourage hostile takeovers

Shark Repellents – In many cases, a company will make special amendments to its charter or bylaws that become active only when a takeover attempt is announced or presented to shareholders with the goal of making the takeover less attractive or profitable to the acquisitive firm.
Also known as a "porcupine provision". Some examples of shark repellents are poison pills, scorched earth policies, golden parachutes and safe harbor strategies

1. Poison Pill- The target company attempts to make its stock less attractive to the acquirer. There are two types of poison pills:
a. A "flip-in" allows existing shareholders (except the acquirer) to buy more shares at a discount.
b. The "flip-over" allows stockholders to buy the acquirer's shares at a discounted price after the merger.

2. Macaroni Defense – The company issues a large number of bonds with the condition they must be redeemed at a high price if the company is taken over. Why is it called Macaroni Defense? Because if a company is in danger, the redemption price of the bonds expands like Macaroni in a pot!

3. Scorched Earth Policy- An anti-takeover strategy that a firm undertakes by liquidating its valuable and desired assets and assuming liabilities in an effort to make the proposed takeover unattractive to the acquiring firm. The scorched earth policy is actually a classic military strategy: generals would instruct troops to burn any land/crops/trees as they retreated so there would be no supplies to refresh the advancing army.

4. Jonestown Defense- A strategy by which the target company engages in an activity that might actually ruin the company rather than prevent the hostile takeover. Also known as a "suicide pill." The term refers to the 1978 Jonestown massacre, where a religious cult (the People's Temple) led by Jim Jones committed mass suicide in Guyana

5. Lobster Trap- In a lobster trap, the company passes a provision preventing anyone with more than 10% ownership from converting convertible securities into voting stock. (Examples of convertible securities include convertible bonds, convertible preferred stock, and warrants.)

6. People Pill- Management threatens that, in the event of a takeover, the entire management team will resign. This is a variation of the poison pill defense.

Related terms –
Sleeping Beauty – A company that is prime for takeover, because it has large cash reserves, undervalued real estate, or huge potential, but has not been approached by an acquiring company.
Saturday Night Special - A slang term used to refer to a surprise takeover attempt. The term alludes to the fact that many takeover bids are announced over the weekend in order to avoid too much publicity.
Dawn Raid - The action of a firm or investor buying a substantial amount of shares in a company (making it a target firm) first thing in the morning when the stock markets open. This is done by a stock broker acting on behalf of a company. Because the bidding company builds a substantial stake in its target at the prevailing stock market price, the takeover costs are likely to be significantly lower than they would be had the acquiring company first made a formal takeover bid.
Sandbag - A stalling tactic used by management to deter a company that is showing interest in taking them over, in hopes that a more favorable company will take them over.
Bear Hug - An offer made by one company to buy the shares of another for a much higher per-share price than what that company is worth. A bear hug offer is usually made when there is doubt that the target company's management will be willing to sell. By offering a price far in excess of the target company's current value, the offering party can usually obtain an agreement. The target company's management is essentially forced to accept such a generous offer because it is legally obligated to look out for the best interests of its shareholders.
White Knight - A company that makes a friendly takeover offer to a target company that is being faced with a hostile takeover from a separate party.
Black Knight - A company that makes a hostile takeover offer on a target company.
Gray Knight - A second, unsolicited bidder in a corporate takeover. A gray knight enters the scene in order to take advantage of any problems between the first bidder and the target company.
Yellow Knight - A company that was once making a takeover attempt but ends up discussing a merger with the target company.
Lady Macbeth Strategy - A corporate-takeover strategy with which a 3rd party poses as a White Knight to gain trust, but then turns around and joins with the unfriendly bidder.
Source - Investopedia