Mid-term Elections:
Mid-term elections are a type of election that occurs during the middle of a presidential term. These elections are held to elect members of the House of Representatives, as well as some members of the Senate. Mid-term elections are held every two years, and they typically take place in November.

By-elections:
By-elections, also known as special elections, are held to fill a vacant seat in a particular constituency. By-elections can occur for a variety of reasons, such as the resignation or death of a sitting MP. These elections are generally held outside of the regular election cycle and can be called at any time.

Differences:
There are some key differences between mid-term elections and by-elections, such as:

1. Timing: Mid-term elections occur every two years, while by-elections can happen at any time.

2. Scope: Mid-term elections are national in scope, while by-elections are limited to a particular constituency.

3. Purpose: Mid-term elections are held to elect members of Congress, while by-elections are held to fill a vacant seat.

4. Turnout: Mid-term elections generally have higher voter turnout, while by-elections tend to have lower turnout.

5. Political Significance: Mid-term elections are seen as a referendum on the party in power, while by-elections are often seen as a test of the popularity of a particular candidate or party.

Conclusion:
In summary, mid-term elections and by-elections are two different types of elections with distinct purposes and scopes. Mid-term elections occur every two years and are national in scope, while by-elections can happen at any time and are limited to a particular constituency. While both types of elections are important, they serve different purposes and have different political significance.