A unique value is a value that only exists once in a list.

A unique distinct list contains all cell values but duplicates are merged into one distinct cell value.

If your are looking for a unique distinct list array formula, see this blog article:
Create a unique distinct alphabetically sorted list

The following array formula extracts unique values from column B in cell D3 and below:

To enter an array formula, type the formula in a cell then press and hold CTRL + SHIFT simultaneously, now press Enter once. Release all keys.

The formula bar now shows the formula with a beginning and ending curly bracket telling you that you entered the formula successfully. Don't enter the curly brackets yourself.

Update 10th December 2020,  Excel 365 formula:

This is a regular formula, you can read about it here: Extract unique values sorted from A to Z

Explaining formula in cell D3

Step 1 - Identify unique values

The COUNTIF function counts cells in cell range based on a condition or criteria. If the value is equal to 1 then it must be a unique value.

COUNTIF($B$3:$B$21, $B$3:$B$21)=1

becomes

COUNTIF({"Federer, Roger "; "Djokovic, Novak "; "Murray, Andy "; "Davydenko, Nikolay "; "Roddick, Andy "; "Del Potro, Juan Martin "; "Federer, Roger "; "Davydenko, Nikolay "; "Verdasco, Fernando "; "Gonzalez, Fernando "; "Wawrinka, Stanislas "; "Gonzalez, Fernando "; "Blake, James "; "Nalbandian, David "; "Robredo, Tommy "; "Wawrinka, Stanislas "; "Cilic, Marin "; "Stepanek, Radek "; "Almagro, Nicolas "},{"Federer, Roger "; "Djokovic, Novak "; "Murray, Andy "; "Davydenko, Nikolay "; "Roddick, Andy "; "Del Potro, Juan Martin "; "Federer, Roger "; "Davydenko, Nikolay "; "Verdasco, Fernando "; "Gonzalez, Fernando "; "Wawrinka, Stanislas "; "Gonzalez, Fernando "; "Blake, James "; "Nalbandian, David "; "Robredo, Tommy "; "Wawrinka, Stanislas "; "Cilic, Marin "; "Stepanek, Radek "; "Almagro, Nicolas "})=1

becomes

{2;1;1;2;1;1;2;2;1;2;2;2;1;1;1;2;1;1;1}=1

and returns

{FALSE; TRUE; TRUE; FALSE; TRUE; TRUE; FALSE; FALSE; TRUE; FALSE; FALSE; FALSE; TRUE; TRUE; TRUE; FALSE; TRUE; TRUE; TRUE}

Step 2 - Build an array containing rank if list were sorted

The less than sign concatenated with the cell reference in the second argument in the COUNTIF function makes it return the rank number if list were sorted from A to Z.

COUNTIF($B$3:$B$21,"<"&$B$3:$B$21)

becomes

COUNTIF({"Federer, Roger "; "Djokovic, Novak "; "Murray, Andy "; "Davydenko, Nikolay "; "Roddick, Andy "; "Del Potro, Juan Martin "; "Federer, Roger "; "Davydenko, Nikolay "; "Verdasco, Fernando "; "Gonzalez, Fernando "; "Wawrinka, Stanislas "; "Gonzalez, Fernando "; "Blake, James "; "Nalbandian, David "; "Robredo, Tommy "; "Wawrinka, Stanislas "; "Cilic, Marin "; "Stepanek, Radek "; "Almagro, Nicolas "},"<"&{"Federer, Roger "; "Djokovic, Novak "; "Murray, Andy "; "Davydenko, Nikolay "; "Roddick, Andy "; "Del Potro, Juan Martin "; "Federer, Roger "; "Davydenko, Nikolay "; "Verdasco, Fernando "; "Gonzalez, Fernando "; "Wawrinka, Stanislas "; "Gonzalez, Fernando "; "Blake, James "; "Nalbandian, David "; "Robredo, Tommy "; "Wawrinka, Stanislas "; "Cilic, Marin "; "Stepanek, Radek "; "Almagro, Nicolas "})

becomes

COUNTIF({"Federer, Roger "; "Djokovic, Novak "; "Murray, Andy "; "Davydenko, Nikolay "; "Roddick, Andy "; "Del Potro, Juan Martin "; "Federer, Roger "; "Davydenko, Nikolay "; "Verdasco, Fernando "; "Gonzalez, Fernando "; "Wawrinka, Stanislas "; "Gonzalez, Fernando "; "Blake, James "; "Nalbandian, David "; "Robredo, Tommy "; "Wawrinka, Stanislas "; "Cilic, Marin "; "Stepanek, Radek "; "Almagro, Nicolas "}, {"<Federer, Roger ";"<Djokovic, Novak ";"<Murray, Andy ";"<Davydenko, Nikolay ";"<Roddick, Andy ";"<Del Potro, Juan Martin ";"<Federer, Roger ";"<Davydenko, Nikolay ";"<Verdasco, Fernando ";"<Gonzalez, Fernando ";"<Wawrinka, Stanislas ";"<Gonzalez, Fernando ";"<Blake, James ";"<Nalbandian, David ";"<Robredo, Tommy ";"<Wawrinka, Stanislas ";"<Cilic, Marin ";"<Stepanek, Radek ";"<Almagro, Nicolas "})

and returns

{7; 6; 11; 3; 14; 5; 7; 3; 16; 9; 17; 9; 1; 12; 13; 17; 2; 15; 0}

Step 3 - Replace boolean value TRUE with rank number

The IF function returns one value (argument2) if TRUE and another (argument3) if FALSE.

IF(COUNTIF($B$3:$B$21,$B$3:$B$21)=1,COUNTIF($B$3:$B$21,"<"&$B$3:$B$21),"")

becomes

IF({FALSE; TRUE; TRUE; FALSE; TRUE; TRUE; FALSE; FALSE; TRUE; FALSE; FALSE; FALSE; TRUE; TRUE; TRUE; FALSE; TRUE; TRUE; TRUE},COUNTIF($B$3:$B$21,"<"&$B$3:$B$21),"")

becomes

IF({FALSE; TRUE; TRUE; FALSE; TRUE; TRUE; FALSE; FALSE; TRUE; FALSE; FALSE; FALSE; TRUE; TRUE; TRUE; FALSE; TRUE; TRUE; TRUE}, {7; 6; 11; 3; 14; 5; 7; 3; 16; 9; 17; 9; 1; 12; 13; 17; 2; 15; 0},"")

and returns

{""; 6; 11; ""; 14; 5; ""; ""; 16; ""; ""; ""; 1; 12; 13; ""; 2; 15; 0}.

Step 4 - Extract the smallest value

The SMALL function lets you calculate the k-th smallest value in a cell range or array. SMALL( array, k)

SMALL(IF(COUNTIF($B$3:$B$21, $B$3:$B$21)=1, COUNTIF($B$3:$B$21, "<"&$B$3:$B$21), ""), ROWS(D2:$D$2))

becomes

SMALL({""; 6; 11; ""; 14; 5; ""; ""; 16; ""; ""; ""; 1; 12; 13; ""; 2; 15; 0}, ROWS(D2:$D$2))

The ROWS function in the second argument has an expanding cell reference that grows when cell D3 is copied to cells below. This makes the formula return a new value in each cell except if duplicates exist in the list.

SMALL({""; 6; 11; ""; 14; 5; ""; ""; 16; ""; ""; ""; 1; 12; 13; ""; 2; 15; 0}, ROWS(D2:$D$2))

becomes

SMALL({""; 6; 11; ""; 14; 5; ""; ""; 16; ""; ""; ""; 1; 12; 13; ""; 2; 15; 0}, 1)

and returns 0 (zero).

Step 5 - Get position in array

The MATCH function finds the relative position of a value in an array or cell range.

MATCH(SMALL(IF(COUNTIF($B$3:$B$21, $B$3:$B$21)=1, COUNTIF($B$3:$B$21, "<"&$B$3:$B$21), ""), ROWS(D2:$D$2)), COUNTIF($B$3:$B$21, "<"&$B$3:$B$21), 0)

becomes

MATCH(0, COUNTIF($B$3:$B$21, "<"&$B$3:$B$21), 0)

becomes

MATCH(0, {7; 6; 11; 3; 14; 5; 7; 3; 16; 9; 17; 9; 1; 12; 13; 17; 2; 15; 0}, 0)

and returns 19.

Step 6 - Return value

The INDEX function returns a value based on row number (and column number if needed)

INDEX($B$3:$B$21, MATCH(SMALL(IF(COUNTIF($B$3:$B$21, $B$3:$B$21)=1, COUNTIF($B$3:$B$21, "<"&$B$3:$B$21), ""), ROWS(D2:$D$2)), COUNTIF($B$3:$B$21, "<"&$B$3:$B$21), 0))

becomes

INDEX($B$3:$B$21, 19)

and returns "Almagro, Nicolas " in cell D3.

Get Excel *.xlsx file

Extract-a-unique-list sorted A to Z from a column-in-excel.xlsx