Here's a breakdown:

* Arrhenius definition: A base is a substance that produces hydroxide ions (OH-) when dissolved in water.

* Brønsted-Lowry definition: A base is a substance that accepts a proton (H+). This definition is broader and includes substances that don't necessarily contain hydroxide ions.

* Lewis definition: A base is an electron pair donor.

Examples of common bases include:

* Metal hydroxides: NaOH (sodium hydroxide), KOH (potassium hydroxide), Ca(OH)₂ (calcium hydroxide)

* Ammonia (NH₃): This acts as a base by accepting a proton from water to form ammonium ions (NH₄+) and hydroxide ions.

* Many organic compounds: For example, amines like methylamine (CH₃NH₂) can accept a proton to form a positively charged ammonium ion.

Let me know if you'd like to explore any of these concepts in more detail!