Partial order approach

Let ${\displaystyle A}$ be a set with a partial order ${\displaystyle \,\leq ,\,}$ and let ${\displaystyle x,y\in A.}$ An element ${\displaystyle m}$ of ${\displaystyle A}$ is called the meet (or greatest lower bound or infimum) of ${\displaystyle x{\text{ and }}y}$ and is denoted by ${\displaystyle x\wedge y,}$ if the following two conditions are satisfied:

1. ${\displaystyle m\leq x{\text{ and }}m\leq y}$ (that is, ${\displaystyle m}$ is a lower bound of ${\displaystyle x{\text{ and }}y}$).
2. For any ${\displaystyle w\in A,}$ if ${\displaystyle w\leq x{\text{ and }}w\leq y,}$ then ${\displaystyle w\leq m}$ (that is, ${\displaystyle m}$ is greater than or equal to any other lower bound of ${\displaystyle x{\text{ and }}y}$).

The meet need not exist, either since the pair has no lower bound at all, or since none of the lower bounds is greater than all the others. However, if there is a meet of ${\displaystyle x{\text{ and }}y,}$ then it is unique, since if both ${\displaystyle m{\text{ and }}m^{\prime }}$ are greatest lower bounds of ${\displaystyle x{\text{ and }}y,}$ then ${\displaystyle m\leq m^{\prime }{\text{ and }}m^{\prime }\leq m,}$ and thus ${\displaystyle m=m^{\prime }.}$^[2] If not all pairs of elements from ${\displaystyle A}$ have a meet, then the meet can still be seen as a partial binary operation on ${\displaystyle A.}$^[1]

If the meet does exist then it is denoted ${\displaystyle x\wedge y.}$ If all pairs of elements from ${\displaystyle A}$ have a meet, then the meet is a binary operation on ${\displaystyle A,}$ and it is easy to see that this operation fulfills the following three conditions: For any elements ${\displaystyle x,y,z\in A,}$

1. ${\displaystyle x\wedge y=y\wedge x}$ (commutativity),
2. ${\displaystyle x\wedge (y\wedge z)=(x\wedge y)\wedge z}$ (associativity), and
3. ${\displaystyle x\wedge x=x}$ (idempotency).

Joins are defined dually with the join of ${\displaystyle x{\text{ and }}y,}$ if it exists, denoted by ${\displaystyle x\vee y.}$ An element ${\displaystyle j}$ of ${\displaystyle A}$ is the join (or least upper bound or supremum) of ${\displaystyle x{\text{ and }}y}$ in ${\displaystyle A}$ if the following two conditions are satisfied:

1. ${\displaystyle x\leq j{\text{ and }}y\leq j}$ (that is, ${\displaystyle j}$ is an upper bound of ${\displaystyle x{\text{ and }}y}$).
2. For any ${\displaystyle w\in A,}$ if ${\displaystyle x\leq w{\text{ and }}y\leq w,}$ then ${\displaystyle j\leq w}$ (that is, ${\displaystyle j}$ is less than or equal to any other upper bound of ${\displaystyle x{\text{ and }}y}$).

Universal algebra approach

By definition, a binary operation ${\displaystyle \,\wedge \,}$ on a set ${\displaystyle A}$ is a meet if it satisfies the three conditions a, b, and c. The pair ${\displaystyle (A,\wedge )}$ is then a meet-semilattice. Moreover, we then may define a binary relation ${\displaystyle \,\leq \,}$ on A, by stating that ${\displaystyle x\leq y}$ if and only if ${\displaystyle x\wedge y=x.}$ In fact, this relation is a partial order on ${\displaystyle A.}$ Indeed, for any elements ${\displaystyle x,y,z\in A,}$

• ${\displaystyle x\leq x,}$ since ${\displaystyle x\wedge x=x}$ by c;
• if ${\displaystyle x\leq y{\text{ and }}y\leq x}$ then ${\displaystyle x=x\wedge y=y\wedge x=y}$ by a; and
• if ${\displaystyle x\leq y{\text{ and }}y\leq z}$ then ${\displaystyle x\leq z}$ since then ${\displaystyle x\wedge z=(x\wedge y)\wedge z=x\wedge (y\wedge z)=x\wedge y=x}$ by b.

Both meets and joins equally satisfy this definition: a couple of associated meet and join operations yield partial orders which are the reverse of each other. When choosing one of these orders as the main ones, one also fixes which operation is considered a meet (the one giving the same order) and which is considered a join (the other one).

Equivalence of approaches

If ${\displaystyle (A,\leq )}$ is a partially ordered set, such that each pair of elements in ${\displaystyle A}$ has a meet, then indeed ${\displaystyle x\wedge y=x}$ if and only if ${\displaystyle x\leq y,}$ since in the latter case indeed ${\displaystyle x}$ is a lower bound of ${\displaystyle x{\text{ and }}y,}$ and since ${\displaystyle x}$ is the greatest lower bound if and only if it is a lower bound. Thus, the partial order defined by the meet in the universal algebra approach coincides with the original partial order.

Conversely, if ${\displaystyle (A,\wedge )}$ is a meet-semilattice, and the partial order ${\displaystyle \,\leq \,}$ is defined as in the universal algebra approach, and ${\displaystyle z=x\wedge y}$ for some elements ${\displaystyle x,y\in A,}$ then ${\displaystyle z}$ is the greatest lower bound of ${\displaystyle x{\text{ and }}y}$ with respect to ${\displaystyle \,\leq ,\,}$ since

$${\displaystyle z\wedge x=x\wedge z=x\wedge (x\wedge y)=(x\wedge x)\wedge y=x\wedge y=z}$$

and therefore ${\displaystyle z\leq x.}$ Similarly, ${\displaystyle z\leq y,}$ and if ${\displaystyle w}$ is another lower bound of ${\displaystyle x{\text{ and }}y,}$ then ${\displaystyle w\wedge x=w\wedge y=w,}$ whence

$${\displaystyle w\wedge z=w\wedge (x\wedge y)=(w\wedge x)\wedge y=w\wedge y=w.}$$

Thus, there is a meet defined by the partial order defined by the original meet, and the two meets coincide.

In other words, the two approaches yield essentially equivalent concepts, a set equipped with both a binary relation and a binary operation, such that each one of these structures determines the other, and fulfill the conditions for partial orders or meets, respectively.